neural tube defects 
 ( ntds ) are a group of common and serious congenital 
 malformations that result from the failure of the neural tube to close 
 by the 28th day postconception . fetuses 
 with anencephaly , a major anterior ntd subtype , are often stillborn 
 or die shortly after birth , whereas fetuses with spina bifida , another 
 major ntd subtype , are usually liveborn , but most often have significant 
 life - long disabilities . birth defects are the leading 
 cause of infant mortality and physical disabilities , and ntds are 
 among the most common form of birth defects , with a prevalence of 
 one in every 1000 established pregnancies worldwide . it has been estimated that more than 320  000 infants 
 worldwide are affected by ntds each year . although folic acid supplementation has been proved to be effective 
 in the prevention of ntds by both observational studies and randomized 
 trials , the underlying mechanism of ntd 
 prevention with folic acid remains poorly understood . meanwhile , it is also widely appreciated that maternal / embryonic 
 exposure to environmental pollutants , especially polycyclic aromatic 
 hydrocarbons ( pahs ) during critical periods of gestation , are associated 
 with increased risks for ntds in offspring . pahs are 
 emitted from the incomplete combustion of fossil fuels 
 or biomass and have potent adverse effects on human health . animal studies have shown that benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide , 
 an important metabolic derivative of benzo(a)pyrene , can cause a variety 
 of malformations including : exencephaly ( the mouse equivalent of human 
 anencephaly ) , gastroschisis and phocomelia in mice . it was reported that shanxi province ranked among the highest 
 pah emission density in china . epidemiological 
 studies have suggested that maternal pah exposure , determined by job 
 title or information elicited by questionnaires is associated with 
 an increased risk of ntds . in a rural population 
 in shanxi province in northern china , maternal exposure to coal used 
 for residential heating and cooking was linked to an increased ntd 
 risk . preliminary studies conducted by 
 our group found higher concentrations of pahs in the venous blood 
 of 35 women who had pregnancies affected by ntds , when compared to 
 18 women who delivered full term healthy infants . in addition , we found an association between higher concentrations 
 of pahs in placental tissue and an elevated risk for ntds . until now , published reports concerning 
 a potential association 
 between maternal pah exposure , as indicated by internal exposure markers , 
 and fetal ntd risk are scarce . having hypothesized that maternal exposure 
 to pahs increases the risk of ntds in offspring , we conducted a case - control 
 study to examine the association between levels of pahs in maternal 
 serum and the risk of ntds , and to identify factors regarding household 
 energy usage and life styles that may be associated with pah concentrations 
 in maternal serum . briefly , subjects were recruited from taigu , pingding , xiyang , shouyang , 
 and zezhou counties , and changzhi city in shanxi province in northern 
 china from november 2010 to march 2013 . the counties in which the 
 studies were conducted are considered to fall within the mean level 
 of socioeconomic development in shanxi province . when a woman with an ntd - affected pregnancy ( including 
 live birth , still birth , or pregnancy termination ) was confirmed as 
 a case , a woman who delivered a term healthy infant at the same birthing 
 hospital was selected to serve as a control , matched to the case by 
 the same county or city of residence , and last menstrual period . information 
 was collected using face - to - face interviews by trained local health 
 workers prior to discharge ( within the first week of delivery or pregnancy 
 termination ) . the questionnaire included information on 
 maternal age ( < 25 , 2529 , or > 29 years old ) , occupation 
 ( farmer or nonfarmer ) , education (  primary or lower  , 
  junior high  , or high school or above  ) , 
 gravidity ( 1 , 2 or > 2 times ) , parity ( 1 , 2 or > 2 times ) ; reproductive 
 history ( yes or no ) , periconceptional folate supplementation ( yes 
 or no ) , fever or flu during early pregnancy ( yes or no ) , alcohol drinking 
 ( yes or no ) , active or secondhand smoking during the periconceptional 
 period ( yes or no ) , separate kitchen from living room / bedroom ( yes 
 or no ) , primary fuel used for cooking ( soft coal , hard coal , biomass , 
 or liquefied gas ) , and conception during the heating season ( yes or 
 no ) . if a conception occurred during the heating season , the following 
 questions were asked : using a stove for heating ( yes or no ) , stove 
 in the living room / bedroom ( yes or no ) , primary fuel used for heating 
 ( soft coal , hard coal , biomass , or liquefied gas ) , frequency of ventilation 
 in the living room / bedroom ( every day , 16 times per week , 
 < 1 time per week ) , and frequency of combustion exhausted gas in 
 living room ( every day , 16 times per week , < 1 time per 
 week ) . fasting blood samples from 117 ntd case women ( anencephaly , 
 44 ; spina bifida , 67 ; encephalocele , 6 ) and 121 control women were 
 collected at delivery or termination of ntd - affected pregnancies with 
 a 4 ml bd vacutainer ( becton , dickinson and company , usa ) . maternal 
 serum was separated within 1 h of collection , temporarily stored at 
 20 c , and then transferred on dry ice to our laboratory 
 for further processing , storage and analyses . the serum was stored 
 in a 2 ml freezing tube from greiner bio - one gmbh co. , germany , which 
 was checked and determined to be free of any detectable pah background . the study protocol was approved by the institutional review board 
 of peking university , and signed consent was obtained from all the 
 recruited subjects . the extraction and 
 cleanup procedures 
 for pah analysis have been described elsewhere and the detailed information about methods and reagents 
 used in this study is provided in the supporting information . twenty - seven 
 parent pahs from j&k chemical , usa including acenaphthylene ( acy ) , 
 acenaphthene ( ace ) , fluorene ( fle ) , phenanthrene ( phe ) , anthracene 
 ( ant ) , fluoranthene ( flu ) , retene ( ret ) , pyrene ( pyr ) , benzo(c)phenanthrene 
 ( bcp ) , cyclopenta(c , d)pyrene ( ccdp ) , benz(a)anthracene ( baa ) , chrysene 
 ( chr ) , benzo(b)fluoranthene ( bbf ) , benzo(k)fluoranthene ( bkf ) , benzo(e)pyrene 
 ( bep ) , benzo(a)pyrene ( bap ) , perylene ( per ) , indeno(1,2,3-cd)pyrene 
 ( icdp ) , dibenz(a , h)anthracene ( daha ) , benzo(g , h , i)perylene ( bghip ) , 
 dibenzo(a , c)pyrene ( dbacp ) , dibenzo(a , l)pyrene ( dbalp ) , dibenzo(a , e)fluoranthene 
 ( dbaef ) , coronene ( cor ) , dibenzo(a , e)pyrene ( dbaep ) , dibenzo(a , i)pyrene 
 ( dbaip ) , and dibenzo(a , h)pyrene ( dbahp ) were analyzed . mass spectrometer 
 ( agilent 7890 , usa ) coupled with a mass spectrometry ( agilent 5975 , 
 usa ) instrument equipped with a hp-5ms capillary column ( 30 m  
 0.25 mm  0.25 m ) . two procedure blanks and a reagent 
 blank were included for each batch ( 1820 samples ) of serum 
 samples . the pah concentration of each sample was calculated by the 
 measured pah concentration subtracting the mean of their corresponding 
 operation blanks . if the pah concentrations of samples were lower 
 than their operation blanks , they were set to zero . the recoveries 
 of 2-fluorobiphenyl [ surrogate standard for low - molecular - weight pahs 
 ( l - pahs ) with 23 benzene rings ( l - pah ) including any , ace , 
 fle , phe , ant , flu , and ret , and p - terphenyl d14 [ surrogate standard for high - molecular - weight pahs ( h - pahs ) 
 with 45 benzene rings including pyr , baa , chr , bbf , bkf , and 
 bap ] were 65.1  28.2% and 79.3  18.7% , respectively . to 
 evaluate the extraction efficiency of the pahs , a serum sample was 
 divided into six aliquots and 27 pahs were spiked with three of them . recoveries of the 27 pah standards varied from 67.4% ( dbaef ) to 106% 
 ( phe ) with mean and standard deviation being 84.9  9.62% . the case - control status of the samples 
 was masked to the person involved in chemical analysis . triglycerides 
 and cholesterol contents were measured by the oxidative method ( intec 
 ( xiamen ) technology co. , ltd , china ) following the standard protocol . 
 the optical density was quantified by an automatic biochemical analyzer 
 ( olympus au400 , japan ) . pah concentration 
 in serum was 
 normalized with lipid content and reported as ng / g lipid . g / l ) of serum was calculated from the concentrations 
 of triglycerides ( tg , g / l ) and total cholesterol ( tc , g / l ) using the 
 formula tl = 0.92 + 1.31  ( tg + tc ) . the mean and standard deviation of the lipid content of the 238 
 maternal samples was 7.33  1.86 the low limits of detection 
 ( lod ) of all pahs in serum ranged from 0.05 to 0.10 ng / ml and the 
 pah concentrations of the 238 serum samples in the final extracts 
 were all within the calibration ranges . concentrations of pahs 
 in serum were not normally distributed , so the median with interquartile 
 range ( iqr ) was used to describe their skewed distributions . differences 
 in concentrations of pahs between cases and controls were evaluated 
 using the mann maternal demographic 
 information between the cases and the controls was compared using 
  test , or fisher s exact test if cell expectation 
 was less than five . pah concentration quartiles of the controls were 
 used as the cutoff values in dose  response analysis . the risk 
 of ntds associated with pah concentrations was estimated by odds ratio 
 ( or ) with 95% confidence internal ( ci ) . the study was originally 
 designed as a loosely matched case - control 
 study . however , some blood samples were not available for evaluation 
 because consent could not be obtained from some subjects , or collecting 
 a blood sample from a control mother matched to a terminated case 
 woman was often delayed because we had to wait until there was an 
 appropriate term delivery for use as the control . to maximize the 
 sample size , we had to separate the pairs and used an unconditional 
 logistic model in odds ratio analyses . serum pah concentration , dichotomized with the 
 median of pah in control group as the cutoff , was used as the independent 
 variable . exposed  was defined as when the pah concentration 
 was above the median , and eleven factors were selected as 
 potential covariates including maternal occupation , age , educational 
 level , body mass index ( bmi ) , previous birth defect history , gravidity , 
 parity , periconceptional folic acid supplementation , active or secondhand 
 smoking , alcohol drinking during the periconceptional period , and 
 fever or flu during early pregnancy . factors that are associated with 
 both the risk of ntds and levels of any individual pahs in maternal 
 serum were adjusted for in the unconditional logistic model with  backward  
 method for further screening covariates . in dose response or 
 trend analyses , pah concentration quartiles of the controls were used 
 as the cutoff values . . a two - tail p value of < 0.05 
 was taken as the statistical significance level . detailed information on the demographic 
 characteristics of this population was previously described and a brief summary description can be found 
 in table 1 . the distribution of maternal age , 
 bmi , education , parity , periconceptional folate supplementation , fever 
 or flu status during early pregnancy , and active or passive smoking , 
 was statistically different between the cases and controls . generally , 
 cases were older [ 26 ( iqr : 2230 ) years in cases vs 24 ( iqr : 
 2128 ) years in controls ] and less educated , and had larger 
 bmi [ 23.5 ( iqr : 20.926.6 ) kg / m in cases vs 21.4 
 kg / m ( iqr : 19.523.4 ) in controls ] , and higher 
 parity . a lower proportion of cases ( 45% ) reported periconceptional 
 folate supplementation , compared to controls ( 58% ) . in addition , a 
 higher proportion of cases reported exposure to passive tobacco smoke 
 ( 67% in cases , 26% in controls ) , and had hyperthermia ( fever or influenza ) 
 during early pregnancy ( 54% in cases , 24% in controls ) . median concentrations 
 of polycyclic aromatic hydrocarbons ( pahs ) in maternal serum by population 
 characteristics are compared in table s1 in the supporting information . seven of them were found to be associated 
 with any of the individual pah levels in serum , and selected as the 
 potential covariates for developing logistic regression model including : 
 maternal age , pregnancy bmi , occupation , parity , periconceptional 
 folate supplementation , fever or flu during early pregnancy , and smoking 
 or secondhand smoking . pearson s 
  test , or fisher s exact test if cell expectation 
 was less 
 than 5 . total 
 number may not be equal to the number of cases or controls due to 
 missing or unknown data . for women overall , there were no significant differences in 
 life styles and 
 energy usage characteristics of this population , with the exception 
 of using a stove for heating ( table 1 ) . in 
 the case group , 69% used a stove for residential heating during winter , 
 compared with 47% in control group . some households , irrespective of case or control status , 
 placed a stove in the living room ( or bedroom ) . coal and liquefied 
 gas were the major fuel types for cooking , and firewood was less often 
 used in this area . most of the households kept living rooms ventilated 
 every day during the heating season ( 75% in cases and 76% in control ) , 
 and fewer proportions of them had indoor combustion exhausted gas 
 in the living room every day ( 31% in cases and 17% in controls ) . among the 27 
 measured pahs , 13 had a detection rate of over 65% in all 238 serum 
 samples , including seven l - pahs ( any , ace , fle , phe , ant , flu , and 
 ret ) and six h - pahs ( pyr , baa , chr , bbf , bkf , and bap ) . the median 
 concentrations of the 13 pahs among cases were approximately twice 
 as high as the levels found in controls ( table 2 ) . the sum of l - pahs ( l - pahs ) , h - pahs ( h - pahs ) , and 
 all pahs ( pahs ) in cases were well over twice as high as the 
 concentrations determined in the controls , respectively . differences 
 in pah concentrations were observed for all three ntd subtypes ( table 2 ) . in the anencephaly subtype , the concentrations 
 of the 13 individual pahs , with the exceptions of bbf and bkf , were 
 higher than control values , whereas all of the 13 individual pahs 
 of the spina bifida subtype group were higher than the controls . there 
 were no differences for all pahs between the anencephaly and spina 
 bifida subtypes . for the encephalocele subtype , the limited sample 
 size ( six subjects ) did not permit meaningful comparisons ; however , 
 the pah concentrations in this subtype were similar to those in other 
 ntd subtypes with only a few exceptions . hence , we chose not to consider 
 encephaloceles separately in the following analyses , including them 
 in the total ntd calculations . iqr , interquartile range ; * p < 
 0.05 , * * p < 0.01 , and * * * p < 
 0.001 , in comparison with the median of controls by mann - whiney u test . the abbreviations 
 of individual 
 pahs are acenaphthylene ( acy ) , acenaphthene ( ace ) , fluorene ( fle ) , 
 phenanthrene ( phe ) , anthracene ( ant ) , fluoranthene ( flu ) , retene ( ret ) , 
 pyrene ( pyr ) , benz[a]anthracene ( baa ) , chrysene ( chr ) , benzo[b]fluoranthene 
 ( bbf ) , benzo[k]fluoranthene ( bkf ) , benzo[a]pyrene ( bap ) . l - pahs 
 is the sum of any , ace , fle , phe , ant , flu and ret ; h - pahs 
 of pyr , baa , chr , bbf , bkf and bap ; pahs of all pahs . overall , all individual pahs 
 above the median concentration of 
 the controls were associated with elevated risks of ntds ( table 3 ) . h - pahs conferred higher ntd risks than 
 l - pahs , which was confirmed by taking h - pahs and l - pahs 
 together in a binary logistic regression model [ e.g. , 4.14 ( 95%ci : 
 2.048.42 ) of h - pahs vs 1.22 ( 95%ci : 0.622.40 ) 
 of l - pahs for total ntds in table s2 of the supporting information ] , whereas the highest or for any individual 
 pahs was observed for ant , a l - pah , in either the unadjusted or adjusted 
 model . the overall direction and magnitude of the ors of all the individual 
 pahs for each subtype were similar . for the anencephaly subtype ( table 3 ) , only three ( ace , ant , and baa ) of the 13 pahs 
 were associated with an elevated risk in both unadjusted and adjusted 
 models , with h - pahs having stronger associations than l - pahs . 
 for the spina bifida subtype ( table 3 ) , all 
 pahs except any , were associated with elevated risks for ntds in the 
 unadjusted model , whereas five of the h - pahs and two l - pahs were associated 
 with elevated ntd risks in the adjusted model . again , h - pahs 
 had more significant or values than l - pahs for the risk of 
 the spina bifida subtype . or , odds ratio ; iqr , interquartile 
 range ; * p < 0.05 , * * p < 
 0.01 , and * * * p < 0.001 . the abbreviations of individual 
 pahs are acenaphthylene ( acy ) , acenaphthene ( ace ) , fluorene ( fle ) , 
 phenanthrene ( phe ) , anthracene ( ant ) , fluoranthene ( flu ) , retene ( ret ) , 
 pyrene ( pyr ) , benz[a]anthracene ( baa ) , chrysene ( chr ) , benzo[b]fluoranthene 
 ( bbf ) , benzo[k]fluoranthene ( bkf ) , benzo[a]pyrene ( bap ) . l - pahs 
 is the sum of any , ace , fle , phe , ant , flu and ret ; h - pahs 
 of pyr , baa , chr , bbf , bkf and bap ; pahs of l - pahs and 
 h - pahs . adjust for maternal 
 general characteristics 
 and exposure , including bmi , parity , fever of flu during early pregnancy , 
 and active or passive smoking and drinking . the association between higher concentrations of l - pahs , 
 h - pahs , and pahs with the risk of ntds indicated a clear 
 dose when the lowest quartile was used as the referent group , increases 
 in the risk of total ntds of 1.66 ( 95% ci : 0.446.23 ) , 3.96 
 ( 95% ci : 0.9915.8 ) , and 6.39 ( 95% ci : 2.0719.8 ) were 
 observed for women whose serum concentrations of pahs were 
 in the second , third , and fourth quartiles , respectively . the detailed 
 data about the risks by quartile of controls for total ntds and its 
 two subtypes can be found in table s3 ( supporting 
 information ) . quartiles of polycyclic aromatic hydrocarbons in maternal 
 serum 
 and risk of total neural tube defects ( ntds ) in a population in shanxi 
 province , china , 20102013 . l - pahs ( left panel ) is the 
 sum of acenaphthylene , acenaphthene , fluorene , phenanthrene , anthracene , 
 fluoranthene , and retene ; h - pahs ( middle panel ) of pyrene , 
 benz[a]anthracene , chrysene , benzo[b]fluoranthene , benzo[k]fluoranthene , 
 and benzo[a]pyrene ; pahs ( right panel ) of l - pahs and 
 h - pahs . serum pah concentrations 
 in subgroups of different population life 
 styles and energy usage characteristics are shown in table s4 ( supporting information ) . overall , no significant 
 differences were found between subgroups of primary fuel type for 
 heating and cooking , isolation between living room ( or bedroom ) , kitchen 
 and stove location , heating by stove , ventilation in the living room , 
 and seasonal time of conception . in 
 this study , we examined the relationships between pah concentrations 
 in maternal serum and ntd risk in their offspring , as well as the 
 association of maternal life style variables and energy usage characteristic 
 with pah concentrations in a population in northern china . overall , 
 both individual and the total pahs in maternal serum were associated 
 with elevated risks for ntds . no association between pah concentrations 
 in maternal serum and life styles and energy usage characteristics 
 were found to be significant . two previous epidemiological studies 
 suggested that maternal exposure to pahs or indoor coal combustion 
 was associated with an elevated risk of ntds in their offspring . one 
 study conducted in the united states found that maternal occupational 
 exposure to pahs may be associated with increased risk of spina bifida 
 in offspring of women who are either normal weight or underweight . another study conducted in the same population 
 as the present study found that indoor combustion , including cooking 
 and heating during the periconceptional period , is associated with 
 an elevated risk of ntds . however , the 
 exposure estimate based on questionnaire or job title may be subject 
 to recall bias or inaccuracy . in addition , questionnaire or job title - based 
 estimation is unable to include other sources of exposure , such as 
 that which occurs via food preparation and consumption . the present 
 study utilized actual pahs determinations in maternal blood as markers 
 of maternal internal pah exposure from all sources , providing a much - needed 
 quantification of exposure lacking in other earlier studies . we found that pahs are significantly associated with elevated ntd 
 risks in unadjusted or adjusted models , with the exception of ret 
 in the adjusted model . ret is usually used as a marker for softwood 
 combustion , and less than 2% of the households 
 of this population used firewood for either cooking or heating . overall , 
 our findings were consistent with the results of our previous pilot 
 study in a smaller population in shanxi province , china . we also compared the pah concentrations in placenta 
 and found that the concentrations of the detected eight pahs in ntd 
 cases were significantly higher than in the controls from a previous 
 study . risks of spina 
 bifida associated with maternal serum pahs were overall higher than 
 those of anencephaly in each quartile of pah concentrations . similarly , only an association between elevated 
 risk of spina bifida in offspring and maternal occupational exposure 
 to pahs was observed a large population - based case - control study in 
 the united states , while no association was observed for the anencephaly 
 and encephalocele . oxidative stress induced damage 
 has been implicated as an underlying 
 mechanism of ntd development related to maternal diabetes and high 
 glucose , and a number of drugs that prenatal exposure leads to ntds . animal studies showed that benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide , 
 a highly reactive intermediate of bap , can induce exencephaly in mice . in this study , the h - pahs were shown to 
 have higher or values than l - pahs . it may be because five of 
 the six detected h - pahs are carcinogenic , and their higher potency 
 may result in more oxidative stress damage than that observed in the 
 l - pahs , which in turn , produces a higher 
 risk for ntds in the exposed offspring . however , this remains only a hypothesis that needs to be mechanistically 
 tested in future studies . coal was the major energy source for 
 heating and cooking , whereas 
 biomass was rarely used in this population . . a higher proportion of heating by using a stove was observed in 
 ntd cases compared to controls . our analysis revealed that coal combustion 
 via stove use may be an important contributor to indoor pah exposure 
 in households of ntd cases , especially during the heating seasons . however , we found no statistical differences in maternal serum pah 
 concentrations between women whose households were heated with a stove , 
 and those households that do not use a stove . no association was observed 
 between maternal pah concentrations and life style or energy usage 
 characteristics including primary fuel used for cooking or heating , 
 separating kitchen from living room / bedroom , conception during the 
 heating season , and ventilation frequency of the living room . indoor 
 burning of solid fuel increases indoor air pah concentrations and 
 population external inhalation exposure risk and internal pah exposure markers should show an increase . this 
 discrepancy may be caused by the contribution of pahs from other pathways , 
 like outdoor inhalation exposure and food ingestion . it has been reported 
 that the main exposure to pahs for general population is food in various 
 countries , which is more than 90% . in future 
 studies , information on daily activities and dietary characters of 
 individual subjects , the corresponding pah pollution in outdoor air , 
 and food should also be determined . this study has many strengths , 
 including the largest number of 
 subjects of its kind in investigating maternal pah exposure and fetal 
 ntd risk with the use of internal exposure biomarkers . the concentrations 
 of 27 pahs with 26 fused benzene rings were comprehensively 
 checked in the maternal serum . first , blood samples were collected 
 at delivery or termination of ntd - affected pregnancies from mothers 
 and the measured pah concentrations may not be fully representative 
 for those in the first month postconception , which is the critical 
 period of neural tube development . further 
 studies are needed to address the relevance of serum pah concentrations 
 at delivery to those during the periconceptional period . second , covariates 
 included in the analyses rely on self - reported information through 
 questionnaire interview , which are subject to recall error . prior 
 to the onset of the study , we trained local health workers at hospitals 
 and clinics to standardize the interview procedures . in addition , 
 we structured the questionnaire such that it contained questions that 
 are straightforward and unambiguous . we performed an independent survey 
 on the accuracy and reproducibility of the case - control data for the 
 major variables , and found the coincidence rate was over 90% . third , the older mothers 
 may have higher concentrations of pahs in their serum resulting from 
 bioaccumulation . thus these limitations 
 suggest the need for investigating the interaction between trace pollutants 
 and risk of ntds . our case - control study revealed that the maternal exposure to pahs 
 is associated with an increased risk of ntds , and h - pahs may render 
 a higher risk than l - pahs . this study adds to the existing evidence 
 of pahs adverse effect on human health , and highlights the 
 need to reduce pahs emission and maternal pah exposure from before 
 pregnancy to protect fetuses who are vulnerable to environmental pollutants . further studies aiming to replicate the findings in other populations 
 and to elucidate the underlying mechanism between pah exposure and 
 ntd development are needed .
polycyclic aromatic hydrocarbons 
 ( pahs ) are ubiquitous environmental 
 pollutants , and have been reported to be a risk factor for human neural 
 tube defects ( ntds ) . 
 we investigated the relationship between pah 
 concentrations in maternal serum and ntd risk in offspring using a 
 case - control study design , and explored the link between pah concentrations 
 to household energy usage characteristics and life styles . 
 one hundred 
 and seventeen women who had ntd - affected pregnancies ( cases ) and 121 
 women who delivered healthy infants ( controls ) were recruited in northern 
 china . 
 maternal blood samples were collected at pregnancy termination 
 or at delivery . 
 twenty - seven pahs were measured by gas chromatography  mass spectrometry . 
 the concentrations of 
 13 individual pahs detected were significantly higher in the cases 
 than in the controls . 
 clear dose 
 response relationships between 
 concentrations of most individual pahs and the risk of total ntds 
 or subtypes were observed , even when potential covariates were adjusted 
 for . 
 high - molecular - weight pahs ( h - pahs ) showed higher risk than low - molecular - weight 
 pahs ( l - pahs ) . 
 no associations between pah concentrations and indoor 
 life styles and energy usage characteristics were observed . 
 it was 
 concluded that maternal exposure to pahs was associated with an increased 
 risk of ntds , and h - pahs overall posed a higher risk for ntds than 
 l - pahs .