Calculation of PFOS and PFOA as the Sum of Isomers
Whereas we directly measured and reported serum total PFOS and PFOA prior to 2013, beginning with NHANES 2013-14 we began measuring linear and branched isomers of serum PFOS and PFOA and summing them to arrive at total PFOS and PFOA. This calculated sum of PFOS (or PFOA) isomers is comparable to the total PFOS (or PFOA) that we reported previously.
The decision to measure linear and branched isomers relates to the processes historically used to produce PFOS and PFOA. From the early 1950s to the early 2000s, electrochemical fluorination (ECF) was the primary process used to synthesize these compounds; thereafter, fluorotelomerization became the primary method. While ECF yielded linear and branched isomers, fluorotelomerization produced almost exclusively linear compounds. Measurement of the two types of isomers is informative because branched isomers may be eliminated from the body more rapidly than linear isomers (Benskin et al, 2009a). In some cases, measurement of the two isomers provides information that is helpful for evaluating exposure source contributions (Benskin et al, 2010, 2009b).
The value of PFOS is calculated as the sum of branched PFOS isomers (perfluoromethylheptane sulfonate isomers, Sm-PFOS) plus the linear isomer (n-PFOS). PFOA is calculated as the sum of branched PFOA isomers (Sb-PFOA) plus the linear isomer (n-PFOA). For each chemical (PFOS and PFOA), the sum of the branched and linear isomers is calculated for each individual NHANES participant. If the value for any isomer is below the limit of detection (LOD), then the imputed value is used. The imputed value is calculated as the LOD divided by the square root of two (Hornung and Reed, 1990) and imputed values are provided in the NHANES public release file. If both isomers for calculating either PFOS or PFOA are reported as less than the LOD, then two imputed values are summed. These calculated PFOA and PFOS results can be used to compare with previous measurements and examine trends in the U.S. population.
The geometric means, selected percentiles, and confidence intervals are calculated using sums with detectable values of 60 percent or higher. If the detection frequency for a category is less than 60 percent, this is indicated by an asterisk (*). Starting with NHANES 2013-14, there is no limit of detection (LOD) for PFOA and PFOS because these values are a calculated sum.
The branched isomers of PFOS (Sm-PFOS) and PFOA (Sb-PFOA) are a mixture of isomers. The isomers known to be included in Sm-PFOS are perfluoro-3-methylheptane sulfonate, perfluoro-4-methylheptane sulfonate, perfluoro-5-methylheptane sulfonate, and perfluoro-6-methylheptane sulfonate. The isomers known to be included in Sb-PFOA are perfluoro-3-methylheptanoic acid, perfluoro-4-methyheptanoic acid, perfluoro-5-methyheptanoic acid, perfluoro-6-methyheptanoic acid, perfluoro-4,4-dimethylhexanoic acid, perfluoro-5,5-dimethylhexanoic acid, perfluoro-3,5-dimethylhexanoic acid, and perfluoro-4,5-dimethylhexanoic acid. The linear isomers of PFOS (n-PFOS) and PFOA (n-PFOA) are single chemicals. Data tables are also presented for each of the four isomers that were measured.
Benskin JP, Hold A, Martin JW. 2009a. Isomer-specific biotransformation rates of a perfluorooctane sulfonate (PFOS) precursor by cytochrome P450 isozymes and human liver microsomes. Environ Sci Technol 43:8566-8572.
Benskin JP, DeSilver AO, Martin LJ, Arsenault G, McCrindle R, Riddell N, et al. 2009b. Disposition of perfluorinated acid isomers in Sprague-Dawley rats: part 1: single dose. Environ Toxicol Chem 28:542-554.
Benskin JP, DeSilver AG, Martin JW. 2010. Isomer profiling of perfluorinated substances as a tool for source tracking: a review of early findings and future applications. Rev Environ Contamin Toxicol 208:111-160.
Hornung RW, Reed LD. 1990. Estimation of average concentration in the presence of nondetectable values. Appl Occup Environ Hyg 5(1):46-51.