methylmercury 
 ( mehg ) exposure adversely affects neurodevelopment , can reduce cognitive function , and may impair 
 cardiovascular health . accurately 
 characterizing mehg exposures and associated health risks requires 
 a detailed understanding of both the amounts and types of seafood 
 consumed by individuals because mehg concentrations can vary by more 
 than 2 orders of magnitude across fish species . dietary surveys that characterize seafood consumption are 
 commonly used to assess mehg exposure but are affected by a variety 
 of uncertainties and provide no information about the environmental 
 origins of mehg in fish . determining the impacts of environmental 
 change on human health requires linking environmental mehg production 
 directly to human exposures . here we explore the utility of hg stable 
 isotope ratios measured in human hair to discern and quantify mehg 
 exposure from different types of seafood as a first step in this linkage . all seven stable isotopes of hg ( 196204 amu ) undergo mass - dependent 
 fractionation ( mdf ) . prior studies in 
 human populations report a consistent 2 increase in 
 hg values between fish and hair of fish consumers that may result from preferential in vivo demethylation 
 of lighter hg isotopes . in contrast to humans , laboratory experiments and field studies 
 on fish suggest that no mdf occurs during trophic transfer between 
 the food that fish consume and the mehg in their muscle tissues . variability in hg values among different fish 
 species reflects differences in microbially mediated inorganic hg(ii ) 
 reduction and mehg degradation as well as photochemical reactions 
 prior to the mercury entering the food web . mass - independent fractionation ( mif ) of the odd - mass - number 
 isotopes 
 of hg ( hg and hg ) occurs primarily during 
 photochemical reactions involving inorganic hg species and mehg . mif is reported as the deviation of a measured isotope ratio from 
 the ratio theoretically predicted to result from mdf . positive mif 
 has been observed in many aquatic organisms , including plankton , fish , 
 and marine mammals largely due to the photochemical degradation of 
 mehg in water . this isotopic signature is retained during trophic transfer of mehg , 
 both through the aquatic food web and into human consumers . previous studies have revealed differences in mif in fish due to 
 differing magnitudes of photochemical degradation of mehg in the water 
 column . as a result , mif signatures can be used 
 to distinguish among fish living in different environments and to 
 characterize human mehg exposure sources . this study investigates the utility 
 of hg isotopes for identifying 
 human exposures to mehg from different seafood sources at an individual 
 level . we use unique samples from individuals with a known homogeneous 
 source of mehg exposure ( pilot whales and human hair from whalers 
 in the faroe islands ) to test the hypothesis that a consistent 2 
 increase in hg values is found in human hair 
 compared to consumed seafood . we then evaluate the consistency between 
 seafood consumption patterns estimated from dietary recall data and 
 from observed isotopic compositions of hair for individuals with more 
 complex dietary mehg exposure . to do this , we use a priori data on fish consumption preferences of anglers from the gulf of 
 mexico to identify different characteristic types of seafood consumers 
 ( e.g. , predominantly coastal fish , ocean fish , or shellfish ) and compare 
 dietary recall information to observed hg isotope ratios . we analyzed hg isotope ratios in archived hair samples collected 
 in 1998 from six members of the faroese whaling society with elevated 
 mehg exposure [ mean hair total hg = 20.6  9.58 g / g ( table 
 s1 of the supporting information ) ] . we 
 chose individuals with high hair total hg concentrations to minimize 
 the quantities of sample needed for hg isotope ratio analysis and 
 isolate exposures from pilot whales ( globicephala melas ) . pilot whales have been hunted and 
 consumed in the faroe islands for more than 500 years and represent the predominant mehg exposure source for 
 individuals who consume their meat . individuals 
 considered here receive virtually all of their mehg exposure from 
 pilot whale , as discussed by grandjean et al . dietary survey data for the general population suggest more than 
 80% of mehg exposure is from pilot whale meat ( mean whale total hg 
 = 3.3 g g ) and that adults consume on average 
 12 g day . in contrast , reported 
 consumption ( 72 g day ) of ocean fish ( mainly cod , gadhus morhea ) by faroese individuals is higher by weight 
 but constitutes less total mehg exposure because of lower hg concentrations 
 in the fish ( mean cod total hg = 0.07 g g ) . given that the faroese whalers are overwhelmingly 
 exposed to mehg from pilot whales , we used this population to more 
 clearly characterize hg isotope fractionation between humans and their 
 diet . samples of muscle tissue from nine male pilot whales were 
 obtained from specimens harvested in 2006 and archived by the faroese 
 museum of natural history . total hg concentrations in these tissues 
 ranged from 2.8 to 5.7 g / g of wet weight [ mean = 4.0  
 0.96 g / g ; n = 9 ( table s1 of the supporting information ) ] . we do not expect that 
 the hg isotopic composition of pilot whale tissue varied significantly 
 between 1998 and 2006 because pilot whales harvested by the faroese 
 show site fidelity to ice - free deep waters beyond the continental shelf and shifts 
 in hg photochemistry due to global changes 
 have likely had minimal impact on their habitat . collected detailed 
 dietary recall data and corresponding hair samples from gulf of mexico 
 anglers who consume highly varied seafood diets . as expected , total 
 hg concentrations in hair samples from gulf of mexico anglers considered 
 here [ mean = 1.2  0.87 g / g ; n = 15 ( table 
 s1 of the supporting information ) ] are 
 much lower than those measured in hair from the faroese whalers who 
 are exposed to mehg mainly from pilot whales . the dominant fraction 
 of mehg exposure for anglers is from fish harvested from the gulf 
 of mexico ( 74% ) , with additional exposure from the commercial 
 market and other locally caught seafood . to identify the isotopic composition of hg in the seafood 
 eaten by gulf of mexico anglers , we considered 
 four coastal fish species ( red snapper , gray snapper , red drum , and 
 speckled trout ) and two oceanic fish species ( yellowfin tuna and blackfin 
 tuna ) . these fish were previously analyzed 
 for total hg concentrations and isotopic compositions by senn et al . , who identified distinct differences in hg isotopic 
 composition between oceanic and coastal fish . on average , oceanic 
 fish displayed 1 higher hg values 
 and 1.5 higher hg values than 
 coastal fish . several other studies have 
 analyzed hg isotope ratios in shellfish and freshwater fish from the 
 gulf of mexico region , including shrimp harvested from the gulf and largemouth bass collected in central florida 
 lakes . pilot 
 whale muscle tissue samples were freeze - dried for 24 h ( virtis sentry 
 freezemobile 12sl ) and then ground to a fine powder using an alumina 
 ball mill mixer ( spex sampleprep mixer / mill ) as described previously . after being ground , 200 mg of fish tissue 
 or 15150 mg of hair per sample was weighed into a 
 ceramic boat and covered with hg - free activated alumina and sodium 
 carbonate powders . the hg in these samples 
 was thermally released in a two - stage furnace using previously published 
 methods and trapped in an oxidizing 1% kmno4 solution . the hg in these solutions was then transferred 
 into secondary kmno4 solutions to separate it from any 
 other combustion products . mercury concentrations 
 were measured in the final solutions by atomic absorption spectrometry 
 and were used to calculate total hg concentrations in the muscle tissue 
 ( wet weight ) and hair samples ( table s1 of the supporting information ) . mercury standards ( nist srm 3133 ) 
 measured during these concentration analyses were reproducible within 
 5% standard deviation ( sd ) . total hg concentrations in replicate 
 samples had a mean relative percent difference ( rpd ) of 6.4  
 1.1% ( n = 7 ) . human hair standards and blanks were 
 processed using the same methods ( see the supporting 
 information for further details ) . hg isotope ratios were 
 measured in samples and standards using multicollector inductively 
 coupled plasma mass spectrometry ( mc - icp - ms ) according to established 
 methods . during each analytical session , sample analytical uncertainty for each isotope ratio was estimated 
 as the larger of 2 times the sd of the same ratio measured either 
 in the relevant procedural standards or in the um - almadn secondary 
 standard . the hg isotopic compositions of samples are reported using 
 delta notation ( hg ) , which is 
 the per mil (  ) deviation from a national institute of standards 
 and technology ( nist ) standard reference material ( srm 3133):1mdf is reported here using hg , as suggested elsewhere . mif is calculated 
 as the difference between the measured hg value and the theoretical value predicted on the basis of 
 kinetic mdf . mif is reported using capital delta notation as2where  
 is equal to 0.252 for hg and 0.752 for hg . isotopic differences between 
 the pilot whales and faroese whalers 
 hair samples were assessed using two sample t tests 
 ( r version 2.14.1 , r development core team ) . the hg/hg ratios in human hair and consumed seafood 
 were analyzed using york regression , which incorporates error in both 
 the dependent and independent variables . we used the quantitative relationship between the hg isotopic composition 
 of faroese whalers hair and seafood consumed ( i.e. , pilot 
 whales ) to construct a deterministic two - end - member isotope mixing 
 model for gulf of mexico anglers . results of this analysis provide 
 an isotope - based estimate of dietary mehg exposure from oceanic and 
 coastal fish species . we compare these results to dietary recall data 
 on seafood consumption for the same individuals . figure 1 shows hg isotope 
 ratios measured in all hair , fish , and whale samples considered in 
 this study . experimental data and previous 
 field studies indicate that mif does 
 not occur during trophic transfer in humans , fish , or marine mammals . hg values of pilot whale tissues were very consistent 
 ( mean = 1.19  0.03 ; n = 9 ) , suggesting 
 similarity in diet and foraging areas . values of hg measured in faroese whalers hair ( mean = 1.27  0.03 ; n = 6 ) were indistinguishable from those of the pilot whales . hg (  ) and hg 
 (  ) values in human hair ( triangles ) and seafood ( times signs ) 
 from the faroe islands and gulf of mexico regions . hair samples of 
 faroese whalers ( black ) have hg values on average 
 1.75 ( 95% confidence interval , 1.502.00 ) higher 
 than those of pilot whales . data for gulf of mexico coastal ( yellow ) 
 and oceanic fish ( red ) are from ref ( 23 ) . isotopic compositions of freshwater fish ( blue ) 
 and shrimp ( pink ) are from refs ( 22 ) and ( 34 ) , respectively . senn et al . found higher mif 
 in oceanic 
 fish than in coastal species from the gulf of mexico ( figure 1a ) . we also measured high positive hg values in hair from anglers who reported consuming predominantly 
 oceanic fish ( figure 1a ) . values for these 
 two individuals ( hg = 2.11 and 1.81 ) 
 were in the same range as those of the oceanic fish { mean = 1.74  
 0.48 } , reinforcing the hypothesis 
 that mif does not occur during trophic transfer . similarly , hg values for the mixed seafood consumers fell in an intermediate 
 range [ 0.831.65 ( figure 1a ) ] 
 between those of coastal and oceanic fish ( figure 1a ) . hair from the two freshwater fish consumers measured here 
 showed low hg values ( 0.49 and 0.81 ) 
 that were similar to those measured in fish from some low - clarity 
 shallow freshwater lakes in florida ( figure 1b ) . hair from two shellfish consumers 
 ( figure 1b ) exhibited hg values ( 0.96 and 1.01 ) slightly higher than those of shrimp and coastal fish from the gulf of mexico . a discrepancy in our findings is 
 apparent for anglers who reported 
 primarily consuming coastal fish from the gulf of mexico . hair from 
 those individuals exhibited hg values ( 1.001.94 ) 
 higher than expected on the basis of those measured in coastal fish [ 0.370.72 ( figure 1a ) ] , suggesting an oceanic source of mehg . we attribute this 
 anomaly to a combination of possible misreporting in dietary recall 
 and differences in the sources of the mehg entering coastal food webs , 
 as discussed further below ( section 4 ) . the hg/hg ratio in hair 
 of gulf of mexico anglers { slope = 1.19  0.03 [ standard error 
 ( se ) ] } observed here is the same as that previously observed in marine 
 fish from the gulf of mexico ( slope = 1.19 ) . experimental and field studies in coastal and open ocean environments suggest that hg/hg ratios between 1.20 and 1.36 result from 
 photochemical degradation of mehg before it is incorporated into food 
 chains . our results suggest this signature is conserved in both hair 
 and seafood , supporting the premise that mif is mainly due to photochemical 
 mehg degradation . on the basis of the results of previous 
 studies , we hypothesized 
 that mdf between seafood 
 and human hair would result in 2 higher hg values in hair . we found that hair from the faroese whalers 
 displayed hg values ( hg 
 = 3.31  0.22 ; n = 6 ) 1.75 higher 
 than those of pilot whales ( hg = 1.55  
 0.21 ; n = 9 ) ( figure 1a ) . hair from gulf of mexico anglers 
 who consumed predominantly oceanic fish also showed a similar offset 
 in hg ( 2.30 and 1.98 ) compared to values 
 measured in oceanic fish from the gulf of mexico ( figure 1a ) . for the five anglers who consumed a mixture 
 of coastal and oceanic fish , average hair hg 
 values were 1.40 higher than those of oceanic fish and 2.35 
 higher than those of coastal fish ( figure 1a ) . for other types of seafood consumers , the hg values in hair were less consistently offset from those observed 
 in seafood from the gulf of mexico . values in hair from coastal fish 
 consumers were offset by an average of 2.27 compared to those 
 of coastal fish from the gulf of mexico , with the exception of one 
 outlier ( 3.20 offset ) . the two freshwater fish consumers had 
 hair hg values of 1.46 and 1.77 , which 
 are approximately 2 higher than those measured in florida 
 largemouth bass ( figure 1b ) . hg values from the two shellfish 
 consumers ( 2.52 and 3.22 ) were much higher than that of gulf 
 of mexico shrimp ( 0.48  0.05 ) ( figure 1b ) . we developed an empirical two - end - member isotope 
 mixing model ( eqs 3 and 4 ) using the results of the faroese analysis ( mdf = 1.75 in 
 hg between human hair and diet ) . we used this 
 model to estimate the fraction of mehg exposure ( fmif and fmdf ) resulting from 
 the consumption of oceanic fish by gulf of mexico anglers , based on 
 hg isotope ratios measured in their diet and hair . we did not include 
 shellfish or freshwater fish consumers in this analysis because we 
 do not have sufficient data on hg isotopes in these organisms to parameterize 
 the model.34where fmif and fmdf represent the 
 expected fractions of mehg 
 exposure from oceanic fish based on observed hg and hg values in hair , respectively , and 
 hgh and hgh are the measured hg isotope ratios in hair from gulf of mexico 
 anglers . the reported average hg isotope ratios in oceanic ( hgoc and hgoc ) and coastal ( hgc and hgc ) fish from the northern gulf of mexico are 
 as follows : hgoc = 0.41 , hgoc = 1.74 , 
 hgc = 0.53 , and hgc = 0.54. we used dietary 
 recall data to develop a comparable estimate of the fraction of an 
 individual s mehg exposure from oceanic fish using eq 5 . the summed product of individual consumed masses 
 ( m ) of oceanic ( oc ) and coastal ( c ) fish species 
 reported over a period of 3 months and their respective mehg concentrations 
 ( c ) were used to estimate the fraction of mehg from 
 ocean fish consumed by each angler . the 3 month recall period represents the same exposure window 
 recorded in hair samples analyzed for hg isotopes.5 figure 2 compares the fraction of individual 
 mehg exposure from oceanic fish consumption based on isotope concentrations 
 ( fmif and fmdf ) and dietary survey recall data ( fr ) . the sensitivity of fmif and fmdf to the observed variability in the fish hg isotope 
 ratios ( 1 sd ) is shown by error bars in figure 2 . this analysis shows that for oceanic fish consumers , there 
 is good agreement between the estimated fraction of mehg from oceanic 
 fish based on the isotope mixing model and the dietary recall data . 
 for anglers who consumed both coastal and oceanic fish , there is also 
 good agreement for three individuals ( < 15% difference ; mixed 2 , 
 3 , and 5 ) but greater discrepancies for the other two anglers ( mixed 
 1 and 4 ) . full results for individuals and details of sensitivity 
 analyses are provided in table s2 of the supporting 
 information . fraction of total mehg exposure due to consumption of 
 oceanic fish 
 for gulf of mexico anglers based on dietary recall data ( black ) , hair 
 hg values ( cross - hatched ) , and hair hg values ( dotted ) . error bars for estimates show variability 
 that can be attributed to the 1 sd variability in fish hg isotope 
 ratios . for the coastal fish consumers , modeled mehg exposures that can 
 be attributed to oceanic fish are consistently higher than dietary 
 recall data . hg values for some coastal fish 
 consumers ( coastal 1 , 3 , and 4 ) are more consistent with those of 
 oceanic fish ( figures 1 and 2 and table s2 of the supporting information ) . results of the sensitivity analysis ( figure 2 and table s2 of the supporting information ) indicate that moderate variability in the hg values of coastal gulf of mexico fish can not explain the high 
 hg values observed in the hair of these anglers 
 ( coastal 1 , 3 , and 4 ) . instead , we attribute differences to a combination 
 of uncertainty in dietary recall data and diversity in the sources 
 of the mehg entering coastal food webs , as discussed further below . recently , blum et al . reported a relationship between fish feeding depth and hg values that provides observational evidence of the uptake 
 of mehg produced in subsurface open ocean seawater by fish . the authors 
 found that hg values in oceanic fish were related 
 to feeding depth and the magnitude of photochemical degradation of 
 mehg in the water column . here we extend this analysis to consider 
 the source of mehg for pilot whales around the faroe islands ( figure 3 ) . values of hg (  ) for pilot 
 whales ( blue 
 arrow ) from this study and nine species of marine fish ( yellow circles ) 
 that feed at different depths in the central north pacific ocean . also shown is the depth preference for the main 
 prey item of pilot whales ( squid , todarodes sagittatus ) . we find that hg values measured in the pilot 
 whales are comparable to those for deep - sea fish ( > 400 m ) from 
 the 
 north pacific ocean ( i.e. , 12 ) 
 ( figure 3 ) . this is consistent with pilot whales 
 predominantly feeding in deep waters beyond the continental shelf . these whales follow the abundance of their primary 
 prey ( squid , todarodes sagittatus ) 
 to depths between 400 and 700 m ( figure 3 ) . our findings thus suggest that mehg in the whales 
 has been produced in the subsurface ocean water column . prior studies have reported 
 offsets in hg 
 values between seafood and consumers similar to those observed in 
 this study . examples include bolivian indigenous populations ( offset 
 = 2.0 ) , a french urban population 
 ( offset = 2.2 ) , and north american 
 dentists ( offset = 1.9 ) . similarities 
 across multiple populations suggest that the mechanism of mdf within 
 the human body is relatively consistent , possibly because of in vivo degradation of mehg , and hg 
 values may be used to identify sources of mehg exposure for individuals . pharmacokinetic variability in mehg dynamics may partially explain 
 differences in the hg offsets between human hair 
 and seafood among recreational anglers from the gulf of mexico ( figure 1 ) . marine mammals such as seals and some whales 
 have the ability to demethylate mehg in their liver and also to exhibit hg values higher than those of prey in their diet . in humans variability in 
 efficiencies of demethylation pathways in the human body would likely 
 follow differences in other pharmacokinetic processes such as intestinal 
 absorption of mehg and partitioning of mehg between blood and hair , leading to some variability in the offset in 
 hg values between human hair and seafood . additional 
 research is needed to better characterize the demethylation pathways 
 in the human body that are responsible for observed mdf between human 
 hair and consumed mehg in seafood . estimated mehg exposure 
 from oceanic fish for gulf of mexico anglers 
 derived from hair isotope data compares well to dietary survey data 
 for consumers of predominantly oceanic and mixed fish species . given 
 the uncertainties in dietary recall data resulting from erroneous 
 portion sizes , inaccurate meal frequencies , or incorrectly identified 
 seafood species , we do not expect perfect agreement 
 between reported consumption and isotope - based estimates . pilot data 
 for oceanic and mixed seafood consumers reported here suggest that 
 hg isotope ratios provide reasonable empirical estimates of mehg exposure 
 sources and may eventually be used to verify the accuracy of dietary 
 recall data . results from the isotope mixing model seem to overestimate 
 the level of mehg derived from oceanic fish for those individuals 
 consuming predominantly coastal fish species . possible explanations 
 for this departure from expectation include misreporting in dietary 
 recall data . because gulf of mexico oceanic fish have concentrations 
 of mehg much higher than those of coastal fish , consumption of even a few meals of oceanic fish in contrast , consumption 
 of a few meals of coastal fish is less likely to have a substantial 
 impact on hair hg isotope ratios of oceanic fish consumers . this may 
 explain why we observed anomalies only among coastal fish consumers 
 despite the fact that misreporting could occur among all individuals . second , we hypothesize that the mehg in coastal fish from parts 
 of the gulf of mexico and other estuaries may have an origin similar 
 to that of oceanic fish . environmental mehg sources in coastal ecosystems 
 include production in benthic sediments , rivers , and the oceanic water 
 column . the relative importance of each 
 source varies substantially across coastal systems and likely also 
 among fish species . for example , harris et al . noted that 
 some areas of the gulf of mexico are highly influenced by riverine 
 hg inflow , while other areas are affected by large inputs from the 
 atlantic ocean loop current . each of these environmental sources of 
 mehg undergoes different photochemical reactions prior to incorporation 
 into the food web and thus can be expected to display different hg 
 isotope ratios . therefore , coastal fish harvested from the broader 
 gulf of mexico region likely exhibit a range of hg values wider than the range of those measured by senn et al . additional data are needed to test the hypothesis 
 that distinct locations of environmental mehg production and uptake 
 at the base of coastal food webs can be measured using hg isotopes 
 in coastal fish . however , our preliminary results point to the potential 
 utility of hg isotopes for tracking environmental sources of mehg 
 production entering coastal food webs , similar to previous studies 
 in the open ocean . such research would 
 have great value for interpreting the foraging patterns of coastal 
 fish and associated locations of mehg uptake . the hg values measured in hair from anglers 
 who were predominantly shellfish consumers ( figure 1b and table s3 of the supporting information ) are 3.5 higher than recent measurements of hg values in shrimp from the gulf of mexico . this larger offset , compared with the previously 
 observed 2 offset for finfish consumers , may result 
 from the larger and more variable proportion of total hg that is inorganic 
 hg in shellfish compared to finfish . inorganic 
 hg in seafood is known to have hg values lower 
 than those of mehg in the same organism . preferential uptake of mehg and excretion of inorganic hg combined 
 with in vivo demethylation in the human body may 
 lead to the larger offset in hg values observed 
 here among shellfish consumers . additional measurements of the isotopic 
 composition of both inorganic hg and mehg in frequently consumed shellfish 
 species ( e.g. , crab and shrimp ) are needed to confirm these hypotheses . moreover , we expect that shellfish consumed by the gulf of mexico 
 anglers are more variable in isotopic composition than the data presented 
 in figure 1 because more than 90% of shrimp 
 consumed in the united states is imported . in summary , these data demonstrate that no mif , but significant 
 mdf , occurs between consumed seafood and human hair . we measured an 
 offset in hg values between consumed seafood 
 and human hair of 2 , which is consistent with prior 
 studies . the mechanism responsible for this observation may 
 be demethylation of mehg within mammals and warrants further investigation . previous studies have identified differences in hg isotopic composition 
 in human biomarkers that can be used to distinguish between exposure 
 to elemental gaseous hg from dental practices and gold mining and 
 exposure to mehg in seafood . in this study , reasonable 
 consistency between estimated fractions of mehg exposure from oceanic 
 fish based on hair hg isotope ratios and dietary recall data for oceanic 
 and mixed fish consumers suggests that hg isotopes show promise as 
 a tool for estimating different mehg exposure sources . both hg and hg values can be used to differentiate 
 between mehg exposure sources ( e.g. , coastal vs oceanic fish ) . in 
 addition , hg isotopes also show potential for providing a direct measurement 
 of environmental mehg sources taken up by biota .
seafood 
 consumption is the primary route of methylmercury ( mehg ) 
 exposure for most populations . 
 inherent uncertainties in dietary survey 
 data point to the need for an empirical tool to confirm exposure sources . 
 we therefore explore the utility of hg stable isotope ratios in human 
 hair as a new method for discerning mehg exposure sources . 
 we characterized 
 hg isotope fractionation between humans and their diets using hair 
 samples from faroese whalers exposed to mehg predominantly from pilot 
 whales . 
 we observed an increase of 1.75 in 202hg values between pilot whale muscle tissue and faroese whalers 
 hair but no mass - independent fractionation . 
 we found a similar offset 
 in 202hg between consumed seafood and hair samples 
 from gulf of mexico recreational anglers who are exposed to lower 
 levels of mehg from a variety of seafood sources . 
 an isotope mixing 
 model was used to estimate individual mehg exposure sources and confirmed 
 that both 199hg and 202hg values 
 in human hair can help identify dietary mehg sources . 
 variability 
 in isotopic signatures among coastal fish consumers in the gulf of 
 mexico likely reflects both differences in environmental sources of 
 mehg to coastal fish and uncertainty in dietary recall data . 
 additional 
 data are needed to fully refine this approach for individuals with 
 complex seafood consumption patterns .