Epidemiologic Evidence on the Health Effects of Perfluorooctanoic Acid (PFOA) — TEI-tagged view

Kyle SteenlandpersName, Tony FletcherpersName, David A. SavitzpersName (2010date). “Epidemiologic Evidence on the Health Effects of Perfluorooctanoic Acid (PFOA).” Environmental Health Perspectivestitle·j.

Object of study — human health effects of perfluorooctanoic acid (PFOAterm) in the epidemiologic literature

Research question — what the epidemiologic literature shows about PFOAterm and lipids, uric acidterm, chronic disease, organ function and reproduction

Contribution — a reviewterm·mth concluding that the data are insufficient for firm conclusions on any disease of concern

Full text — tagged

Epidemiologic Evidence on the Health Effects of Perfluorooctanoic Acid (PFOAterm) Abstract We reviewed the epidemiologic literature for PFOAterm.

Perfluorooctanoic acid (PFOAterm) does not occur naturally but is present in the serum of most residents of industrialized countries (U.S. median, 4 ng/mL). Drinking water is the primary route of exposure in some populations, but exposure sources are not well understood. PFOAterm has been used to manufacture such products as Gore-Tex and Teflon. PFOAterm does not break down in the environment; the human half-life is estimated at about 3 years. PFOAterm is not metabolized in the body; it is not lipophilic. PFOAterm is not directly genotoxic; animal data indicate that it can cause several types of tumors and neonatal death and may have toxic effects on the immune, liver, and endocrine systems. Data on the human health effects of PFOAterm are sparse. There is relatively consistent evidence of modest positive associations with cholesterolterm·key and uric acidterm, although the magnitude of the cholesterolterm·key effect is inconsistent across different exposure levels. There is some but much less consistent evidence of a modest positive correlation with liver enzymes. Most findings come from cross-sectionalterm·key studies, limiting conclusions. Two occupational cohort studies do not provide consistent evidence for chronic disease; both are limited by sample size and reliance on mortality data. Reproductive data have increased recently but are inconsistent, and any observed adverse effects are modest.

Epidemiologic evidence remains limited, and to date data are insufficient to draw firm conclusions regarding the role of PFOAterm for any of the diseases of concern.

Keywords: epidemiologyterm·key, health effects, PFOAterm Perfluorooctanoic acid (PFOAterm, or C8) is a perfluorinated compoundterm (PFCterm) that does not occur naturally but was introduced in the environment after World War II. PFCsterm are fluorocarbons with at least one additional atom or functional group; PFOAterm has a carboxylic acid group (C 7 F 15 COOH). Most people tested in the United States have PFOAterm in their serum (measured via perfluorooctanoate, the disassociated ion of PFOAterm), with a median of 4 ng/mL, but the source of exposure is not clear ( Calafat et al. 2007b ). PFOAterm has also been detected in the serum of populations in a number of industrialized countries ( Fromme et al. 2009 ; Kannan et al. 2004 ; Lau et al. 2007 ). Limited data suggest that PFOAterm in water systems may result from wastewater treatment plants, which concentrate PFOAterm ( Becker et al. 2008 ; Pistocchio and Loos 2009 ). Levels of PFOAterm in the serum appear to be decreasing in the U.S. population, because some large users have curtailed emissions ( Calafat et al. 2007b ; Olsen et al. 2007b ).

Determinants of PFOAterm in the general population are known to include age (higher at younger and older ages), sex (males higher), and race (whites higher) ( Calafat et al. 2007a , 2007b ; Emmett et al. 2006a ; SteenlandpersName et al. 2009a ). However, none of these attributes are very strong predictors. For those living near a point source of contamination, contamination of drinking water has been identified as the major route of exposure ( Emmett et al. 2006a ; Hölzer et al. 2008 ; SteenlandpersName et al. 2009a ).

PFOAterm has been used in the production of a wide variety of industrial and consumer products, such as Gore-Tex and Teflon. PFOAterm does not break down once in the environment, because of the strong carbon–fluorine bonds, leading to widespread buildup and bioaccumulation. Two studies provide estimates of the half-life for PFOAterm in humans. In a study of 26 retired fluorochemical production workers (mean years retired at initial blood collection, 2.6), followed up for 5 years, the median half-life was 3.4 years ( Olsen et al. 2007a ). In a study of 200 people who had been exposed via public water supplies and followed for 1 year after installation of filtration for the water supplies, the median half-life was 2.3 years ( Bartell et al. 2010 ). The latter estimate suffers from the short follow-up time (although this population will be followed for 5 years), whereas the former suffers from small numbers and a lack of data at the time when exposure ceased.

PFOAterm is not metabolized in the body; its tissue distribution in humans is unknown, but studies in rats suggest it is likely to be present primarily in the liver, kidney, and blood ( Kennedy et al. 2004 ; Lau et al. 2007 ). It is easily absorbed via the gastrointestinal tract in rats. It is not lipophilic, unlike chlorinated hydrocarbons, but binds to serum albumin and is excreted primarily from the kidney. It is not directly genotoxic ( Andersen et al. 2008 ).

There is a considerable amount of animal data on the health effects of PFOAterm, which has been summarized by others [ Kennedy et al. 2004 ; Lau et al. 2007 ; U.S. Environmental Protection Agency (EPA) 2005 ]. The relevance of animal (primarily rodent) data for humans is controversial because of a much shorter half-life in rodents (measured in days) and the possible dependence of some animal toxicity on a peroxisome proliferation mechanism that is likely to be less important in humans (see below).

PFOAterm induces tumors of the testicles, liver, and pancreas in rodents ( Biegel et al. 2001 ; Sibinski 1987 ; U.S. EPA 2005 ) via dietary intake, and there is some evidence it also increases mammary tumors ( Sibinski 1987 ; U.S. EPA 2005 ). The lowest doses at which effects have been observed in these rodent studies appear to be several orders of magnitude higher than human doses from drinking water contaminated at a level of 1 ng/mL.

In its draft risk assessment, the U.S. EPA (2005) concluded that evidence was suggestive that PFOAterm is carcinogenic in humans. In its reviewterm·mth of that risk assessment, three of the four members of the EPA scientific advisory board concluded more strongly that PFOAterm was “likely to be carcinogenic in humans” ( U.S. EPA 2006 ).

PFOAterm also reduces birth weightterm·key in mice and causes neonatal death in rats ( Lau et al. 2007 ; U.S. EPA 2005 ). In mice it decreases the B-cell and T-cell immune responses, and in rats it results in atrophy of the spleen and thymus, causes hepatomegaly, and decreases levels of cholesterolterm·key ( Kennedy et al. 2004 ; Lau et al. 2007 ; Loveless et al. 2006 ). Several experimental studies have reported that PFCsterm impair thyroid hormone homeostasis. Depression of serum triiodothyronine (T 3 ) and/or thyroxine (T 4 ) in PFOA- and PFOS-exposed rats and monkeys has been reported ( Butenhoff et al. 2002 ; Lau et al. 2007 ), but without an expected corresponding elevation of thyroid-stimulating hormone (TSH) through feedback stimulation of the hypothalamic–pituitary–thyroid axis. However, it has been hypothesized these associations could be an artifact of the use of analog methods for assessment of free thyroid hormones and are absent if the equilibrium dialysis method is used ( Chang et al. 2007 ; Lau et al. 2007 ).

One mechanism that has been proposed for these effects in rodents is peroxisome proliferation. In animals, PFOAterm is a strong peroxisome proliferatorterm·dom in the liver, and this proliferation has been shown to alter lipids, liver enzymes, and liver size ( Kennedy et al. 2004 ; Lau et al. 2007 ; Loveless et al. 2006 ). Proxisome proliferation and the resulting activation of a nuclear receptor [peroxisome proliferator-activated receptor α (PPARα)] have also been proposed as a mechanism for tumor induction and for the immune and hormonal changes seen in rodents ( Lau et al. 2007 ). However, it is not known if this mechanism is relevant to humans, where peroxisome proliferation is generally less apparent ( Dewitt et al. 2008 ; Klaunig et al. 2003 ). Furthermore, new observations question whether this mechanism is specifically relevant to liver carcinogenesis in rodents ( Guyton et al. 2009 ). This issue remains a subject of active investigation.

In the United States, the only populations exposed above background levels are occupational cohorts and two communities in Minnesota and West Virginia/Ohio. The two communities have been exposed via water contamination coming from adjacent industrial plants; in Minnesota the mean PFOAterm was 15 ng/mL as measured recently ( Minnesota Department of Health 2009 ), whereas in the Mid-Ohio Valley the mean was 82 ng/mL in 2005 ( Emmett et al. 2006a ; Frisbee et al. 2009 ; SteenlandpersName et al. 2009a ). In the Mid-Ohio Valley population, a great deal of health information and clinical chemistries, as well as serum measurements of PFCsterm, were gathered in a 2005–2006 survey of 69,000 residents of contaminated water districts. Descriptions of these data have been posted on the C8 Health Project web site ( C8 Health Project 2009 ).

PFOAterm is often correlated in human serum with another PFCterm, perfluorooctanesulfonic acid (PFOS). PFOS is also present in most human serum in the United States, with a median serum level of 21 ng/mL, although as with PFOAterm, the sources of this exposure are poorly characterized ( Calafat et al. 2007a ). Until recently PFOS has been used in the manufacture of Scotchgard (3M, St. Paul, MN) among other products (also until recently Scotchgard could emit PFOAterm as it degraded). Its half-life has been estimated at 5.4 years ( Olsen et al. 2007a ). PFOS is also considered an animal carcinogen by U.S. EPA (2005) . The epidemiologyterm·key of PFOS is even more limited than that of PFOAterm. We do not reviewterm·mth epidemiologic studies of PFOS per se here but mention findings for PFOS in studies that consider both PFOAterm and PFOS, to help interpret the PFOAterm results.

Lipids, Uric Acidterm, Diabetes, and Cardiovascular and Cerebrovascular Disease Lipids A positive association of PFOAterm with cholesterolterm·key was observed in six occupational studies ( Costa et al. 2009 ; Olsen and Zobel 2007 ; Olsen et al. 2000 , 2003 ; Sakr et al. 2007a , 2007b ), three studies of a highly exposed community ( Emmett et al. 2006b ; Frisbee et al. in press ; SteenlandpersName et al. 2009b ), and one general population study ( Nelson et al. 2010 ). Six of these studies showed a statistically significant association (at the p = 0.05 level; all references in this article to statistical significance refer to p ≤ 0.05), whereas two occupational studies ( Olsen and Zobel 2007 ; Olsen et al. 2000 ) and the smaller of the two community studies ( Emmett et al. 2006b ) did not. All of these studies were cross-sectionalterm·key except for three: Sakr et al. (2007b) had data on 454 workers with multiple PFOAterm and cholesterolterm·key measurements over an average of 10 years, Costa et al. (2009) had data on 56 workers with such multiple measurements, and Olsen et al. (2003) had data on 174 workers with two measurements. All three of these studies found a significant correlation over time between cholesterolterm·key and PFOAterm levels.

The risk of high cholesterolterm·key (≥ 240 mg/dL) also increased in relation to PFOAterm in some studies. For example, in SteenlandpersName et al. (2009b) , comparing the highest quartile of PFOAterm versus the lowest yielded an odds ratio (OR) for high cholesterolterm·key of 1.38 [95% confidence interval (CI), 1.28–1.50].

Many studies also showed positive relationships of PFOAterm with other lipids (i.e., low density lipoprotein cholesterolterm·key and triglycerides), with the exception of high density lipoprotein cholesterolterm·key, which was consistently not found to be associated with PFOAterm. Several studies ( Frisbee et al. in press ; Nelson et al. 2010 ; SteenlandpersName et al. 2009b ) also showed associations of lipids with PFOS that were similar in magnitude to those with PFOAterm. This finding raises the question of whether all PFCsterm are associated with increased lipids via some common mechanism. PFOAterm and PFOS are generally correlated in the serum; in two large studies, the correlations were 0.31 (in a population with high-PFOA exposure; SteenlandpersName et al. 2009b ) and 0.65 (in a general population with background PFOAterm and PFOS exposures; Nelson et al. 2010 ). Generally, models including both compounds showed an attenuation of the association for both PFOAterm and PFOS, without one being clearly dominant. Only one study ( Olsen et al. 1999 ) addressed the effect of PFOS alone, an occupational cross-sectionalterm·key study that showed a significant positive correlation with cholesterolterm·key at one plant but not at another.

Given the cross-sectionalterm·key nature of most of these studies, it is not clear whether a ) PFOAterm increases cholesterolterm·key and other lipids; b ) both PFOAterm and cholesterolterm·key are jointly affected by some other attribute, substance, or mechanism that causes their correlation; or c ) high lipids might cause increased retention of PFOAterm in the body (reverse causalityterm·mth). Some evidence against this last hypothesis is provided by SteenlandpersName et al. (2009b) , where those who had taken statins, associated with a large decrease in cholesterolterm·key, had PFOAterm levels similar to those who had not taken statins. The three longitudinal studies indicating that cholesterolterm·key and PFOAterm were related over time in subjects with repeated measurements ( Costa et al. 2009 ; Olsen et al. 2003 ; Sakr et al. 2007b ) strengthen the case for a causal relationship, although they do not preclude a relationship in which both PFOAterm and cholesterolterm·key jointly covary with some biologic process that is also changing over time.

The strength of the association of PFOAterm and cholesterolterm·key varied considerably by study, making interpretation even more problematic: The lower the range of PFOAterm that was studied, the greater the change in cholesterolterm·key per unit change in PFOAterm. Thus, the studies of community populations report larger shifts in cholesterolterm·key per unit change in PFOAterm levels than do the occupational studies (which have higher exposures). Table 1 summarizes the changes in cholesterolterm·key associated with changes in PFOAterm. Assuming a linear relationship between the two (not always the case), the slope relating PFOAterm to cholesterolterm·key varied by two to three orders of magnitude. One possibility that might explain some of this discrepancy would be if the slope of an exposure–response relationship was steep at low PFOAterm levels and then flattened out, as might be the case, for example, if some biological pathways were saturated. There is a suggestion of such flattening in some studies ( Frisbee et al. in press ; SteenlandpersName et al. 2009b ). Most studies did not examine the exposure–response curve in detail.

Changes in cholesterolterm·key in relation to changes in PFOAterm levels.

The findings of associations between PFOAterm and increased cholesterolterm·key in humans contradict what would be expected from animal studies, where PFOAterm decreases, not increases, serum lipids ( Lau et al. 2007 ). However, as also noted, it is not known whether peroxisome proliferation, the mechanism of action involved, is relevant to humans, where it is generally less apparent.

Uric acidterm Uric acidterm is a natural product of purine metabolism and has both oxidant and antioxidant properties. There is considerable epidemiologic evidence that elevated uric acidterm is a risk factor for hypertension ( Feig et al. 2008 ; Shankar et al. 2006 ).

Three cross-sectionalterm·key studies report positive associations between PFOAterm and uric acidterm. Two are occupational studies [ Costa et al. 2009 ( n = 160); Sakr et al. 2007a ( n = 1,000)], and one is a community study [ SteenlandpersName et al. 2010 ( n = 55,000)]. No detailed results are provided in Sakr et al. (2007a) . Costa et al. (2009) , using a cross-sectionalterm·key analysis, found mean uric acidterm levels of 6.29 μg/mL for 34 currently exposed workers, versus 5.73 μg/mL for 34 matched nonexposed workers ( p = 0.04). In addition, using repeated measures of both PFOAterm and uric acidterm over a 7-year period to perform a longitudinal analysis of this group ( n = 56), Costa et al. (2009) found a significant positive association between uric acidterm and PFOAterm; the two varied together over time.

In the cross-sectionalterm·key study by SteenlandpersName et al. (2010) , both PFOAterm and PFOS were significantly associated with uric acidterm. An increase from the lowest to highest decile of either PFOAterm or PFOS was associated with an increase of 0.2–0.3 mg/dL uric acidterm. Risk of abnormally high uric acidterm (hyperuricemiaterm·dom, defined as > 6.0 mg/dL for women, > 6.8 mg/dL for men) increased modestly with increasing PFOAterm; the ORs by quintile of PFOAterm were 1.00, 1.33 (95% CI, 1.24–1.43), 1.35 (95% CI, 1.26–1.45), 1.47 (95% CI, 1.37–1.58), and 1.47 (95% CI, 1.37–1.58; test for trend, p < 0.0001). A positive but less pronounced trend was seen for PFOS. Inclusion of both correlated fluorocarbons in the model indicated that PFOAterm was a more important predictor than was PFOS. Inclusion of albumin in the model, which binds with PFOAterm, had little effect on these results.

The overall strength of association between PFOAterm and uric acidterm is modest in both the SteenlandpersName et al. (2010) and Costa et al. (2009) studies. Causality cannot be inferred from these cross-sectionalterm·key data. There are mechanisms by which reverse causalityterm·mth might be applicable. For example, uric acidterm and PFOAterm may share renal transport systems (organic ion transporters 1 and 3) governing excretion of each substance ( Eraly et al. 2008 ; Nakagawa et al. 2008 ), so it is possible that if the levels of PFOAterm increase, the secretion of urate is decreased and therefore blood urate levels may secondarily increase. However, whether this shared transporter hypothesis is relevant in humans remains speculative at this point.

Cardiovascular disease Given the findings for cholesterolterm·key and uric acidterm noted above, it is reasonable to ask whether PFOAterm is associated with cardiovascular disease. The data on cardiovascular disease, however, are limited in quantity and quality, consisting primarily of mortality studies among relatively small cohorts of workers.

The mortality of workers ( n = 6,207) at a DuPont plant using PFOAterm, who were employed between 1948 and 2002, has been studied by Leonard et al. (2008) and Sakr et al. (2009) . Serum PFOAterm levels among all workers at this plant in 2004 averaged 240 ng/mL, with an average of 490 ng/mL among workers in the PFOAterm areas ( Sakr et al. 2007a ). Standardized mortality ratios (SMRs) were calculated using several different referent populations. SMRs for ischemic heart disease (IHD) compared with U.S. and state general populations were < 1.0, as might be expected because of the healthy worker effectterm·mth. Sakr et al. (2009) conducted dose–response analyses of these same data using a job–exposure matrix ( Kreckmann et al. 2009 ), which used job categories and measured serum levels for 1,000 workers in 2004 to assign all cohort members to four quartiles of cumulative serum PFOAterm. The mean serum was 231 ng/mL. Categorical dose–response analyses for IHD using different lags (0. 5, 10, 15, 20 years) and different methods of forming quartiles (based on case exposures or entire cohort exposures) showed no significant positive trends at the p = 0.05 level, but the analysis using a 10-year lag and quartiles formed based on the PFOAterm distribution of the entire cohort did suggest a positive trend [risk ratios (RRs) = 1.0, 1.0, 1.4, and 1.6; p for trend = 0.06].

Workers at the 3M plant in Minnesota are the other principal occupational cohort exposed to PFOAterm. The most recent mortality update of this cohort is from Lundin et al. (2009) ( n = 3,922). Workers were divided into not exposed, probably exposed, and definitely exposed groups based on job category, with ranges of median serum levels in the probably exposed job categories of 300–1,500 ng/mL and in the definitely exposed job categories of 2,600–5,200 ng/mL. Mortality by the three exposure groups showed rate ratios of 1.0, 1.2 [95% confidence interval (CI), 0.9–1.7), and 0.9 (95% CI, 0.4–2.1), with observed deaths of 92, 103, and only 6 in the definitely exposed job categories. Analyses by cumulative weighted years of exposure (weights for unexposed, probably exposed, and definitely exposed groups were 1, 30, and 100) showed rate ratios of 1.0 (138 observed), 1.2 (95% CI, 0.9–1.8; 42 observed), and 0.8 (95% CI, 0.5–1.2; 21 observed). There is little or no evidence of a positive trend in these data. Cumulatively, the two studies provide insufficient data for inferences regarding an association between PFOAterm and cardiovascular disease.

Finally, Melzer et al. (2010) found no trend in self-reported history of heart disease in data from the National Health and Nutrition Examination Survey (NHANES), after dividing PFOAterm serum levels into quartiles. These data are limited by the low range of exposure (background levels in the general population), the cross-sectionalterm·key nature of the data, and the lack of validation of self-reported data.

Cerebrovascular disease Cerebrovascular disease is of interest given the findings for uric acidterm, which is related to blood pressure, a major determinant of cerebrovascular disease. (No studies directly address PFOAterm and blood pressure.) Again, however, the epidemiologyterm·key is limited to the two principal occupational studies described above. Leonard et al. (2008) found a deficit of cerebrovascular mortality in comparing exposed DuPont workers with nonexposed DuPont workers (SMR = 0.86; 95% CI, 0.60–1.20). Lundin et al. (2009) found a suggestion of a positive trend of stroke mortality across nonexposed, probably exposed, and definitely exposed job categories using SMRs [SMRs of 0.5 (95% CI, 0.3– 0.8), 0.7 (0.4–1.1), and 1.6 (0.5–3.7), respectively; no test for trend presented]. Using a cumulative measure of weighted years of exposure, the rate ratios were 1.0 (23 deaths), 0.6 (95% CI, 0.2–2.2; 3 deaths), and 2.1 (95% CI, 1.0–4.6; 9 deaths), for < 1 year, 1–5 years, and > 5 years, respectively. Like the data for heart disease, these data are too sparse to draw any conclusions.

Diabetes Diabetes is not a disease of particular interest based on animal data, but some epidemiologic studies have addressed this end point nonetheless. Leonard et al. (2008) found a deficit of diabetes mortality in comparing exposed DuPont workers with the U.S. population but a 2-fold increase in diabetes mortality compared with other, nonexposed DuPont workers (SMR = 1.97; 95% CI, 1.23–2.98; 22 deaths), arguably a more suitable comparison group because of the healthy worker effectterm·mth. Similarly, in internal comparisons among 3M workers, Lundin et al. (2009) found an excess of diabetes mortality in comparing a combined category of probably and definitely exposed workers and nonexposed workers (RR = 3.7; 95% CI, 1.4–10.1; 18 vs. 5 deaths). This finding was much less pronounced using years of exposure weighted by a qualitative judgment of intensity.

Diabetes mortality is not the optimal measure for a largely nonfatal disease. MacNeil et al. (2009) studied diabetes prevalence in a community exposed to high levels of PFOAterm via contaminated drinking water near a DuPont plant. Self-reported diabetes prevalence (7.8%) and date of onset were collected from 54,468 adults, who had a median serum PFOAterm level of 28 ng/mL (mean = 87 ng/mL) in 2005–2006 at the time of the survey. Overall, there was no relation between diabetes prevalence, either self-reported or validated by medical records, and serum PFOAterm as measured at the time of the survey. A nested case–control analysis was conducted in which the population was restricted to participants who had at least 20 years of continuous residence in a specific water district ( n = 13,922), to maximize the likelihood that serum levels in 2005–2006 would reflect the relative exposure of subjects in the past. Cases were restricted to those validated by medical records and occurring in the 10 years before 2005 ( n = 1,055). There was a negative trend in diabetes occurrence by increasing serum PFOAterm decile (ORs = 1.00, 0.71, 0.60, 0.72, 0.65, 0.65, 0.87, 0.58, 0.62, 0.72). In additional analyses, fasting serum glucose showed no consistent trend by increasing decile of serum PFOAterm.

Cancer Animal data suggest that pancreatic, testicular, liver, and perhaps breast cancer may be related to PFOAterm exposure ( Lau et al. 2007 ). Human studies are again restricted to the two U.S. occupational cohorts, as well as one follow-up study of the general population in Denmark. Leonard et al. (2008) found no excesses for liver, pancreas, testicular, or breast cancer based on small numbers of deaths (8, 11, 1, and 2 respectively). A 2-fold excess of kidney cancer was observed (SMR = 1.81; 95% CI, 0.94–3.16; 12 deaths). Lundin et al. (2009) similarly found no excesses of liver and pancreatic cancers based on small numbers (3 and 13 deaths, respectively) and did not report on testicular or kidney cancer. Internal analyses by job category were reported for three cancers (prostate, pancreas, bladder). Some positive trend for prostate cancer was found by exposure group based on job category [nonexposed, probably exposed, definitely exposed; RRs of 1.0, 3.0 (95% CI, 0.9–9.7), and 6.6 (95% CI, 1.1–37.7), based on 4, 10, and 2 cases respectively. For pancreatic cancer, the corresponding RRs were 1.0 and 1.7 (95% CI, 0.5–4.8), respectively, for nonexposed and for combined probable/definite exposure groups. The data from both occupational cohorts are clearly quite sparse and prohibit any firm conclusions.

There is also a study of 55,053 Danish adults 50–65 years of age who were followed from enrollment in 1993–1997 until 2006 through linkage with the Danish cancer registry ( Eriksen et al. 2009 ). There were 713, 332, 128, and 67 incident cases of prostate, bladder, pancreatic, and liver cancers found in this period. A case–cohort approach was used. Dividing the population into quartiles of serum PFOAterm (based on the cases), no significant linear trends by quartile were seen for any of the four cancers studied, although modest positive associations with prostate and pancreas cancers were reported [RR for highest quartile vs. lowest quartile: prostate cancer, 1.18 (95% CI, 0.84–1.65); pancreatic cancer, 1.55 (95% CI, 0.85–2.80)]. Trends were of borderline significance for prostate cancer and pancreatic cancer and were absent or negative for bladder and liver cancer. Findings for PFOS largely paralleled those for PFOAterm, which is not surprising given that the Spearman correlation between the two was 0.80. This study, although much larger than the occupational studies cited above, had much lower ranges of PFOAterm exposure (mean = 6–7 ng/mL), typical of a general population.

Immune, Thyroid, Liver, and Kidney Function and Sex Hormones Immune function Animal studies have demonstrated that PFOAterm causes lymphoid organ atrophy and decreased de novo antibody production in certain strains of mice ( DeWitt et al. 2008 ; Yang et al. 2000 ). Total doses received in the Dewitt et al. (2008) study appear to have been about an order of magnitude higher than the total lifetime dose received by people drinking water contaminated with 1 ng/mL PFOAterm. There appears to be strain as well as species sensitivity to the immunotoxic effects of PFOAterm, with rats being considerably less sensitive than mice ( Loveless et al. 2008 ), leading some to question the relevance of these data to humans. Immunotoxicity by PFOAterm in rodents appears to occur both through and independent of peroxisome proliferation ( Yang et al. 2000 ). Immunosuppression, at least in part, occurs in exposed mice lacking the PPARα receptor ( DeWitt et al. 2009 ). There is also evidence from animal studies that PFOAterm can suppress inflammatory responses ( Taylor et al. 2006 ), which is similar to the effects of other PPAR agonists ( DeWitt et al. 2009 ).

Human studies to date are very limited and have included only relatively insensitive test measures, such as white blood cell count or serum immunoglobulin (Ig) levels. Emmett et al. (2006a) examined the effects on PFOAterm on hematologic parameters in a cross-sectionalterm·key community study composed of residents who lived in a contaminated water district for at least 2 years and a smaller group of volunteers who met the same eligibility criteria ( Emmett et al. 2006b ). They found no association between serum PFOAterm and lymphocytes, neutrophils, eosinophils, or basophils. There was one small statistically significant association between PFOAterm and absolute monocyte counts; elevated numbers of monocytes are often associated with recent infection, which was not controlled in the analysis.

Costa et al. (2009) measured serum levels of IgG, IgM, and IgA in 34 male workers engaged in the production of PFOAterm. Almost 13% of the workers showed levels of serum IgA outside the laboratory referent values. Because of the small population and lack of controls, it is difficult to draw any conclusions from these data. Furthermore, the levels of serum Igs, which represent circulating antibodies, are extremely variable in the population and relatively insensitive markers for immune system effects.

Overall human studies are insufficient for assessing whether PFOAterm is or is not related to immune suppression.

Thyroid function After experimental evidence of PFCsterm reducing T 3 and T 4 in rats and monkeys ( Butenhoff et al. 2002 ; Lau et al. 2007 ), a number of occupational and community studies have investigated these clinical markers of thyroid function, usually in cross-sectionalterm·key studies. Emmett et al. (2006b) investigated the association between PFOAterm and health outcomes in 371 community residents with high serum levels (median, 534 ng/mL) due to contaminated drinking water. No significant association was found between TSH and the serum level of PFOAterm. In a small community study ( n = 31) of New York State anglers, at much lower exposure levels (PFOAterm serum geometric mean = 1.33 ng/mL), potential associations were investigated between serum concentrations of eight measured PFCsterm and levels of TSH and free T 4 ( Bloom et al. 2010 ). No associations were found, but study power was very low.

Olsen et al. (2003) conducted a cross-sectionalterm·key analysis that included two plants with 255 and 263 workers, respectively. Multivariate regression analysis by quartiles of PFOAterm or PFOS exposure showed no significant association of either compound with T 3 , T 4 , or TSH. A continuous regression suggested a slight positive association between log PFOAterm and log T 3 (coefficient = 0.016, p -value = 0.01); however, the contribution of this association to the variance of the outcome was negligible (partial R 2 = 0.01). A longitudinal analysis of up to three examinations for 174 workers from the same company found no association between PFOAterm and thyroid hormones ( Olsen et al. 2003 ).

In a more recent occupational study, Olsen and Zobel (2007) reported on analyses of thyroid hormones in relation to PFOAterm among 552 employees in three plants (median serum level, 1,100 ng/mL). A log–log regression adjusted for age, body mass index (BMI), and alcohol intake showed a negative slope for free T 4 ( p = 0.01) and positive slope for T 3 ( p = 0.05), with consistent direction of effect for each plant. For total T 4 and TSH, correlations across plants and within plants were not significant.

A recent cross-sectionalterm·key analysis of self-reported thyroid disease in the NHANES in relation to either PFOAterm or PFOS reported a significant association for PFOAterm in females, and a similar but much less precise association in males, but no association for PFOS ( Melzer et al. 2010 ). The OR for females of ever having had thyroid disease for the upper quartile of PFOAterm serum levels was 1.64 (95% CI, 1.09–2.46) compared with the first quartile, and for current thyroid disease plus thyroid hormone medication the equivalent OR was 1.86 (95% CI, 1.12–3.09). For men the respective findings were 1.58 (95% CI, 0.74–3.39) and 1.89 (95% CI, 0.60–5.90). These findings are striking given the very low exposures compared with the occupational studies, but the study suffers from using self-reported diagnoses, having no thyroid hormone data, having no exposure data at the time of diagnosis, and not distinguishing between hypo- and hyperthyroidism. In addition, there is some concern that the disease or treatment of disease may affect serum levels of PFOAterm.

Overall, the occupational data at high exposure levels show no consistent evidence of a reduction of thyroid hormone. In addition associations between T 3 and T 4 measured by the more common analog methods need to be treated with some caution because free T 4 assay results have been reported to be affected by the presence of PFOS, which can lead to apparent positive associations ( Chang et al. 2007 ). It is not known whether this is a problem with PFOAterm as well. Two community studies at lower exposure levels did not indicate an effect, but study sizes were small. The NHANES data show a strong effect on self-reported thyroid disease at very low levels, not easily interpreted in the context of the other studies.

Sex hormones Increases in estradiol and decreases in testosterone with PFOAterm exposure have been observed in rodents ( Lau et al. 2007 ). However, there are very limited human data on hormones in relation to serum PFOAterm, found in three occupational cross-sectionalterm·key studies. Olsen et al. (1998) studied 191 male workers divided into four exposure groups and measured estradiol, 17-hydroxyprogesterone, prolactin, and bound testosterone. No significant associations between exposure and these hormones were found, but most of the population were in the two lowest exposure groups (< 1 and < 10 ng/mL), with only five subjects in the > 30 group. Costa et al. (2009) reported a multivariate regression of estradiol and testosterone in relation to PFOAterm in a group of 56 workers followed in routine occupational surveillance, in which there were no associations of sex hormones with serum PFOAterm. Finally, Sakr et al. (2007a) found a significant association between serum PFOAterm and both estradiol and testosterone in men ( n = 782) in linear regression models (regression coefficients = 22.3 and 0.6, respectively; p -values = 0.017 and 0.034, respectively). Apparently there were no significant findings for hormones in the 243 women.

Liver function Studies of the distribution of PFOAterm in rodents have found relatively high concentrations in the liver. Increased liver weight has been found in rodents and nonhuman primates exposed experimentally to PFOAterm ( Lau et. at 2007 ). In rodents, but not necessarily in primates, liver toxicity is related to the PPARα mode of action. However, the nonhuman primate studies and the evidence of toxicity in PPAR-null mice suggest that liver toxicity via other modes of action may be relevant, including other nuclear receptors in the liver ( Dewitt et al. 2009 ; Ren et al. 2009 ). The animal findings on liver toxicity have prompted a number of studies of liver enzymes in people exposed to PFOAterm.

Emmett et al. (2006b) reported on the association between serum levels and several liver enzymes in serum among 371 community residents who were exposed to PFOAterm (median exposure, 354 ng/mL). Enzymes included alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine aminotransferaseterm (also known as serum glutamic pyruvic transaminaseterm), and gamma glutamyl transpeptidase (GGT). No correlations of serum PFOAterm and enzymes approached statistical significance. One of the comparisons for abnormal values (AST; abnormal not defined) was associated with PFOAterm, but the normal group had higher PFOAterm, suggesting a protective rather than a harmful effect of PFOAterm.

Costa et al. (2009) reported on a surveillance program of workers in a PFOAterm plant. Bilirubin and four liver enzymes showed no associations in cross-sectionalterm·key analyses, but in a longitudinal analysis ( n = 56) all four enzymes indicated a positive slope of increasing enzyme levels with respect to PFOAterm, three with p -values < 0.05 [alanine transaminase (ALT), GGT, and ALP] and one (AST) not significant. Results are presented in terms of regression coefficients for log-transformed variables, which are sometimes hard to interpret in terms of strength of effect. Total bilirubin fell with increasing PFOAterm ( p < 0.01).

Sakr et al. (2007b) , in a longitudinal study of liver enzymes in an occupational population ( n = 454; mean serum PFOAterm = 1,130 ng/mL), found significant associations with AST (0.35 units increase per 1,000 ng/mL PFOAterm; 95% CI, 0.10–0.60). Total bilirubin declined with increasing PFOAterm (0.008 mg/L decline per 1,000 ng/mL; 95% CI, −0.0139 to −0.0021). In each of these cases, the change was small compared with the mean and standard deviation of these clinical markers.

Sakr et al. (2007a) also conducted a cross-sectionalterm·key study at the same plant of PFOAterm, lipids, and liver enzymes, in 1,025 workers with potential exposure to PFOAterm. AST, ALT, and total bilirubin were not significantly associated with the level of PFOAterm, but GGT was significantly positively associated with PFOAterm ( p = 0.02). Observed increases were no longer significant when the analysis was restricted to those not on lipid-lowering medications. No notable change was found for the log of total bilirubin ( Sakr et al. 2007a ).

Several reports by Olsen and colleagues have addressed the liver enzymes AST, GGT, and ALP along with bilirubin in exposed workers; in the most recent and extensive analysis, Olsen and Zobel (2007) reported data from 552 volunteers from three plants (mean serum level = 2,210 ng/mL). For the three plants together, there was a modest positive association for log GGT (coefficient = 0.0326, p = 0.05) and log ALT (coefficient = 0.0249, p = 0.06), in a model with BMI but not in a model with triglycerides, but no evidence of an effect for ALP or AST in either model. However, results were inconsistent across plants, with these overall positive results derived from just of one of the four plants included. For bilirubin, the relationship was consistently negative and significantly negative overall (exposure–response coefficient: in model with BMI, −0.0325, p = 0.001; in model with triglycerides, −0.0267, p = 0.01). These positive findings contrast with earlier cross-sectionalterm·key analyses of these plants that were essentially negative (e.g., Gilliland and Mandel 1996 ; Olsen et al. 1999 ).

Lin et al. (2010) assessed the relationships between ALT, GGT, and bilirubin and four PFCsterm, including PFOS and PFOAterm, measured in serum in the NHANES surveys of 1999–2000 and 2003–2004 ( n = 2,216). They found significant trends for liver enzymes across quartiles of PFOAterm and in regressions of log-transformed enzyme levels against log-transformed PFOAterm. However exposure contrasts were low, with the lowest quartile < 2.9 and the top quartile > 5.95 ng/mL PFOAterm. Given these small exposure contrasts, the models suggested a much larger effect per unit exposure than did other studies.

In summary, studies of liver enzymes have reported some associations with PFOAterm. Four independent studies (two longitudinal and two cross-sectionalterm·key studies) reported reduced bilirubin associated with increasing PFOAterm, and at least two studies showed results suggesting increasing AST or GGT in relation to PFOAterm. However, the changes in liver enzymes are quite small, and the magnitude of effect is quite inconsistent, with results from the NHANES study showing an effect of exposure going from 1 to 3 ng/mL, whereas other studies looked at contrasts of thousands of nanograms per milliliter of PFOAterm, in some cases with no apparent association. These results provide limited evidence of some biological activity of PFOAterm in human liver, the clinical significance of which is uncertain.

Kidney function PFOAterm concentrates in the kidney in animals and therefore is of interest as a potential target organ. However, no animal data document kidney toxicity, and human data are also quite limited.

In an occupational study, Costa et al. (2009) reported no significant association between PFOAterm and either urea nitrogen or creatinine. These negative results were consistent with those of Emmett et al. (2006b) , who found no significant association between either blood urea nitrogen or creatinine with the serum level of PFOAterm.

Reproductive and Developmental Outcomes Toxicology studies indicate the potential for PFOAterm and PFOS to affect fetal growth and development. In utero exposure to PFCsterm is associated with a range of nonspecific adverse developmental outcomes in mouse, rat, and rabbit models ( Lau et al. 2007 ; Olsen et al. 2009 ), including reduced fetal weight and increased neonatal mortality. Extrapolation across species and end points is particularly problematic in reproductive toxicology.

Epidemiologic research on PFOAterm and reproductive end points was initiated quite recently, with most studies published in 2007 or later ( Olsen et al. 2009 ). With few exceptions, the studies examine serum PFOAterm in relation to the risks of a range of different adverse outcomes. Except for fetal growth, most of the end points have been examined in only one or two studies.

Starting with fertility, one study of semen quality and reproductive hormones in men ( Joensen et al. 2009 ) reported decrements in sperm count and number of morphologically normal sperm with higher exposure to the combined level of PFOAterm and PFOS, but weaker associations with PFOAterm alone. An integrated measure of couple fecundity, time to pregnancy, was examined by Fei et al. (2009) . This same population was examined for several other reproductive end points and thus warrants a more extensive description.

Starting with > 100,000 participants in the Danish National Birth Cohort, which recruited women from 1996 to 2002 and followed women through the course of pregnancy and continued with childhood assessment, Fei et al. (2009) randomly selected 1,400 women with singleton live births for measurement of PFOAterm and PFOS in blood collected during the first trimester of pregnancy (mean PFOAterm = 5.6 ng/mL). The evaluation of time to pregnancy showed increased risk of irregular menstrual cycles in the upper three quartiles of PFOAterm relative to the lowest quartile (15.0% vs. 9.0%) and an increase in mean PFOAterm with increasing time required to conceive (5.4 ng/mL for < 6 months to conception, 6.0 ng/mL for 6–12 months to conception, and 6.3 ng/mL for > 12 months to conception). The odds of infertility (≥ 12 months without conception) were elevated in the upper three quartiles of PFOAterm exposure (ORs ranging from 1.6 to 2.5) relative to the lowest quartile. Very similar patterns were reported for PFOS. The absence of dose–response gradients for fertility across levels suggests the possibility of some peculiarity in the lowest exposure group but does not preclude a causal relationship with a maximum effect at a very low exposure level.

The course of pregnancy, including risk of miscarriage and preeclampsia, has been addressed in a study of a subset of women exposed to markedly elevated levels of PFOAterm in the Mid-Ohio Valley ( Stein et al. 2009 ). No association was found between PFOAterm and miscarriage, whereas a weak association was found for preeclampsia (for above-median exposure to PFOAterm: OR = 1.3; 95% CI, 0.9–1.9). Nolan et al. (2009a) used birth certificate information to address pregnancy complications in women residing in a high-PFOA area of the Mid-Ohio Valley and reported extremely imprecise associations with anemia and dysfunctional labor and lower overall risk of labor and delivery complications.

The most extensive set of studies has examined fetal growth, birth weightterm·key, duration of gestation, and related indices of in utero development. Seven studies reported continuous measures of birth weightterm·key in relation to continuous measures of PFOAterm exposure ( Apelberg et al. 2007 ; Fei et al. 2007 ; Hamm et al. 2009 ; Inoue et al. 2004 ; Monroy et al. 2008 ; So et al. 2006 ; Washino et al. 2009). Two studies of background exposure to PFOAterm reported clear evidence of decreased birth weightterm·key in relation to increased PFOAterm ( Apelberg et al. 2007 ; Fei et al. 2007 ), with smaller decrements reported in two other studies of background exposure levels ( Hamm et al. 2009 ; Washino et al. 2009). Magnitude of association ranged from a decrement of 37 to 104 g per log unit increase in PFOAterm exposure, with varying degrees of statistical precision, with one study ( Fei et al. 2007 ) reporting a smaller reduction of 10.6 g per ng/mL PFOAterm. Three smaller studies reported no association between PFOAterm and birth weightterm·key ( Inoue et al. 2004 ; Monroy et al. 2008 ; So et al. 2006 ). Risk of low birth weightterm·key (< 2,500 g) was elevated in the upper three quartiles relative to the first quartile in the analysis of the Danish National Birth Cohort ( Fei et al. 2007 ), but a measure of small-for-gestational-age (which accounts for duration of gestation) was not related to PFOAterm in the Danish National Birth Cohort ( Fei et al. 2007 ) nor in a study from Alberta, Canada ( Hamm et al. 2009 ). An ecologic study of residence and birth weightterm·key in the Mid-Ohio Valley found no association between PFOAterm exposure and birth weightterm·key ( Nolan et al. 2009b ). Stein et al. (2009) did not find an association between PFOAterm and self-reported low birth weightterm·key.

Cumulatively, the studies provide inconsistent suggestions of a possible decrement in birth weightterm·key associated with PFOAterm exposure, with studies varying in whether the association with PFOS is similar ( Apelberg et al. 2007 ), stronger ( Stein et al. 2009 ; Washino et al. 2009), or weaker ( Fei et al. 2009 ; Hamm et al. 2009 ) than that reported for PFOAterm. Different measures of fetal growth across studies make it difficult to assess whether the studies of background PFOAterm and those addressing more elevated PFOAterm levels are consistent with one another, but thus far the positive evidence for an effect on birth weightterm·key comes from studies of background exposure levels. Although an adverse impact of PFOAterm on birth weightterm·key would be of some concern in suggesting sensitivity of the fetus, shifts of the reported magnitudes within the normal birth weightterm·key range are of little or no clinical significance.

Other indices of fetal growth have been examined in isolated studies with suggestions of adverse effects of PFOAterm on head circumference and ponderal index ( Apelberg et al. 2007 ) and on abdominal circumference and birth length ( Fei et al. 2008b ). Studies addressing preterm birth reported elevated risks for intermediate exposure categories only ( Fei et al. 2007 ) or no association ( Hamm et al. 2009 ; Stein et al. 2009 ).

Child health and development have been examined to a very limited extent. Stein et al. (2009) reported an association between PFOAterm levels exceeding the 90th percentile and birth defects in aggregate (OR = 1.7; 95% CI, = 0.8–3.6). Birth defects in aggregate were not associated with residence in a high-PFOA area of the Mid-Ohio Valley in a study based on birth certificates ( Nolan et al. 2009a ). Fei et al. (2008a) examined parent-reported developmental milestones in infants 6 and 18 months of age and found no indication of delays associated with higher PFOAterm exposure.

Discussion Although the epidemiologic literature on the health effects of PFOAterm is growing rapidly, overall it remains limited in volume and quality. Many studies are cross-sectionalterm·key in nature, making causal inference difficult. Other studies are small and have too few outcome events to draw firm conclusions. The range of PFOAterm varies greatly in different populations studied, with some studies of biomarkers identifying effects at low general population levels and others with a much higher and large range of exposures finding smaller or no effects.

The most consistent findings have been for modest increases in cholesterolterm·key, and to a lesser degree for a modest increase in uric acidterm, among those with higher PFOAterm levels. However, these findings have been largely based on cross-sectionalterm·key data (exposure and outcome measured simultaneously), calling into question whether the relationship is causal. Three longitudinal studies show that PFOAterm changes over time correlate with changes in cholesterolterm·key levels ( Costa et al. 2009 ; Olsen et al. 2003 ; Sakr et al. 2007b ), and one longitudinal study shows a similar effect on uric acidterm ( Costa et al. 2009 ). These longitudinal studies strengthen the case for a causal relationship, although they do not necessarily preclude that PFOAterm and cholesterolterm·key, or PFOAterm and uric acidterm, are both associated with some other biological process that is changing over time. Other cross-sectionalterm·key findings show some evidence of an increase in liver enzymes with higher serum PFOAterm, but these findings have not been consistent across studies, and observed enzyme increases have been modest.

Cohorts followed for mortality have the potential to shed more light on the occurrence of chronic disease. There have been only two of these, both occupational ( Leonard et al. 2008 ; Lundin et al. 2009 ). Findings in these two cohorts have not been consistent, with the exception of diabetes mortality, which was significantly elevated in both studies when worker comparison groups were used. However, a large study of diabetes prevalence, a more sensitive outcome, using retrospective exposure estimates, showed no relation with PFOAterm ( MacNeil et al. 2009 ). Other findings from the occupational cohort mortality studies have been inconsistent, with excesses in one but not the other. There is some evidence of increased kidney cancer and heart disease in one cohort ( Leonard et al. 2008 ; Sakr et al. 2009 ), whereas stroke and prostate cancer showed a positive exposure–response trend in the other ( Lundin et al. 2009 ). All these findings (except for the heart disease finding) were based on small numbers of deaths. Another nonoccupational cohort, with much lower exposure levels, has been followed for cancer in Denmark, with no significant excesses observed ( Eriksen et al. 2009 ). In summary then, for chronic disease, sparse and inconsistent evidence prohibits any conclusions.

The research on reproductive and developmental health end points is expanding rapidly and offers sporadic evidence that is difficult to interpret. A range of indicators of fetal development, including birth weightterm·key, may be subtly affected by relatively low levels of PFOAterm, but the potential for an artifactual relationship with PFOAterm metabolism cannot be discounted ( SavitzpersName 2007 ). Furthermore, the associations, although often statistically significant, are quite small in absolute terms and of limited direct clinical significance. Other studies that address more clinically consequential outcomes report null or marginal adverse effects, with none replicated in independent studies at this time.

In summary, the epidemiologic evidence accumulated thus far on health effects of PFOAterm comes principally from two occupational cohorts and several occupational cross-sectionalterm·key studies, as well as community populations with background exposure levels, and one community with elevated exposure in the Mid-Ohio Valley.

Although the occupational studies have the advantage of substantially elevated exposure ranges, they suffer from several limitations. They are primarily limited to males. The cohort mortality data are limited for rare diseases and those that are typically not fatal. The prevalence studies measure subclinical end points, typically blood chemistry, and are limited for causal inference and are of variable clinical significance.

The community studies of populations with background exposures are more broadly representative of the general population, but it is difficult to discern clear associations within the limited exposure range available to study. In contrast to these important but inherently limited sources of epidemiologic information, we are pursuing an unusual opportunity to extend our knowledge regarding the health effects of PFOAterm considerably by conducting a series of studies of an exposed community population in the Mid-Ohio Valley ( Frisbee et al. 2009 ) (see http://www.c8sciencepanel.org for a description of the studies currently under way). Levels of PFOAterm exposure in this population are markedly above background (median values were 5 times background but included a wide range; mean levels were 15 times background), although still below occupational levels (which often have mean levels 200 times background). The population is large (> 60,000 individuals) and comprises a wide age range (including children). In addition, we are also studying disease incidence in a cohort of approximately 6,000 workers with much higher levels (e.g., levels of 1,000 ng/mL, 200 times background). [ Leonard et al. (2008) studied this same cohort for mortality.] Although there are methodologic challenges in this setting as well, particularly in reconstructing historical exposure levels and in ascertaining health end points accurately, these studies should provide new evidence complementary to that which has been generated to this point. To the extent that there are other such communities around the world that are exposed to elevated levels of PFOAterm due to a point source, epidemiologic research in those settings would be of clear value and help to evaluate the consistency of future findings in the Mid-Ohio Valley.

Footnotes References