This study showed similar levels of a urinary biomarker of PAH exposure in summer and winter. Red and processed meat intake and GSTT1-02 polymorphism showed correlations with 1-OHPG levels in winter, while CYP1B1-07 polymorphism had an inverse correlation. In summer, making bread at home, second-hand smoke, and GSTM1-02 polymorphism were correlated with 1-OHPG levels, but GSTP1-02 polymorphism showed an inverse association; food intake data were not available for this season.
Participants in the current study, who were all female non-smokers, had higher 1-OHPG levels compared to non-smokers without occupational exposure to PAHs in several other studies that used the same methodology as this study for assessing 1-OHPG; in fact, the levels in this study were comparable to those of smokers in the other studies (Table ). The higher SD for Cr-adjusted 1-OHPG levels in the current study suggests that inter-individual variability is larger in our study participants than in other populations. Almost all of the participants in the current study used natural gas or kerosene for heating in winter. The similarity of the Cr-adjusted 1-OHPG levels in summer and winter implies that exposure to PAHs through heating in winter does not have a major effect on total exposure in Golestan. As we used paired t-tests, any substantial overall intra-individual variability in the two seasons is unlikely. In concordance with this finding, the fuels used for heating and the duration of their use did not show any association with 1-OHPG levels.
Examples of 1-OHPG levels assessed with the same methodology in different studiesa.
Diet seems to be the main source of PAHs among non-smokers who do not have occupational exposure to those compounds in studies from other regions of the world (Straif et al., 2005
). However, information on the role of diet in exposure to PAHs in Golestan is limited. Only two earlier studies in Golestan have investigated such association. These studies assessed the PAH levels in commonly eaten foods and in bread and rice (the two major staple foods in the region), but they did not find high levels on average (Joint Iran-IARC Study Group, 1977
; Hakami et al., 2008
). In one of those studies, however, when the daily intake of benzo[a
]pyrene was estimated from its levels in cooked rice, bread, and drinking water, the daily intake of the compound in Golestan was higher than in a low-risk area in Iran (Hakami et al., 2008
). In the current study, participants were questioned about the frying intensity of the consumed food in general rather than over the previous 24
h. 1-OHPG levels in urine reflect recent exposure (within hours) to PAHs, so the level may not be an optimal indicator of exposure to PAHs from fried food in general rather than in the previous few hours. This fact and also the association between red meat intake and 1-OHPG level suggests that fried red meat may be an important source of PAH among those who use this food in Golestan, although the association for fried red meat did not reach statistical significance in our study. Frying has been shown to increase the PAH content of foods; for example, in a study in Spain, the total PAH content of raw lamb increased from 5.5
μg/kg fresh weight to 16.9
μg/kg after frying (Perello et al., 2008
). In our study, the association between BMI and 1-OHPG level in winter may be related to food intake, because the association was disappeared following adjustments for other factors, including red meat intake.
Making bread at home in summer showed an association with 1-OHPG levels. On the other hand, there was no association between 1-OHPG and making bread at home in winter. The fuel used for making bread in both seasons was natural gas, so it is not clear why such a practice was associated with increased exposure to PAHs in only one round of our study. Some possible explanations may be that the association is due to chance or that making bread at home is a proxy for another habit or practice which is associated with higher exposure to PAHs in summer than in winter.
Only a limited number of studies have investigated the association between second-hand smoke and PAHs or their metabolites in urine (Scherer et al., 1992
; Suwan-Ampai et al., 2009
). A recent large study reported that elevated urinary concentrations of most PAH metabolites are associated with exposure to second-hand smoke (Suwan-Ampai et al., 2009
). Our findings also suggest a similar association in both seasons, although the association in winter was weaker than in summer. Our small sample size may be one of the reasons for not observing a statistically significant association in winter. There was no increase in 1-OHPG levels with exposure to tobacco smoke from one or two cigarettes; this amount of exposure might not be sufficient to increase 1-OHPG levels in urine.
Phase I enzymes in the metabolic pathways of PAHs, including CYPs and MPO, usually catalyze PAHs to more reactive metabolites. Phase II enzymes, such as GSTs, catalyze the conjugation of PAHs or their reactive metabolites to compounds that are more water-soluble, so that they are more readily excreted (Chen et al., 2007
). Several studies have investigated the association between polymorphisms in genes encoding the above enzymes and levels of PAH metabolites in urine, but the results have not been very consistent (Alexandrie et al., 2000
; Schoket et al., 2001
; Apostoli et al., 2003
; Abnet et al., 2007
; Chen et al., 2007
; Chuang and Chang, 2007
; Petchpoung et al., 2011
). In our study, polymorphisms in some CYP
genes were associated with lower levels of 1-OHPG, while deletions in GSTM1
were associated with elevated levels. However, except for CYP2E1-05
, the other polymorphisms did not show a consistent pattern in winter and summer. The significance and repeatability of these findings need to be examined in further studies.
While the Caspian Littoral region in general is a humid area, the areas with highest incidence of ESCC in Golestan have a relatively dry climate (Mahboubi, 1971
). The urine samples collected in winter were more diluted than summer urine, which may be related to greater dehydration of inhabitants in Golestan in summer than in winter. From a methodological point of view, this suggests the importance of adjustment for creatinine levels when a single spot urine sample is analyzed for PAH metabolites. Different patterns of association in the winter and summer rounds in our study, e.g., with regard to BMI, suggest exposure to different sources of PAH in different seasons in Golestan. These potential variations should also be considered in future investigations.
Our study has strengths and limitations. We collected extensive questionnaire data, which allowed us to examine the associations of interest while considering the influence of several other lifestyle-related factors. We collected samples in two seasons to reduce the effect of inter-individual and seasonal variations. However, not having 24-h food intake data in the second round (summer) did not allow us to compare the pattern of exposure to PAHs in the two seasons with regard to diet and to adjust the analyses in the summer for diet. Furthermore, multiple comparisons were done in our analyses. Therefore, some of the statistically significant results might have arisen by chance. However, at least regarding exposure to red meat intake, the results seem to be robust.
In summary, the study confirms high exposure to PAHs of the general population in Golestan, which does not seem to be related to heating in winter, and for which certain foods may be important factors among individuals who consume those foods. With regard to esophageal cancer, it is possible that drinking hot tea, a habit common in Golestan, exposes esophageal cells to higher amounts of ingested PAHs than in individuals who drink their tea at moderate temperatures (Islami et al., 2009a
). Exposure to second-hand tobacco smoke also showed an association with PAHs, but the prevalence of the exposure was low. Although high-temperature frying is a common cooking practice in Golestan, approximately 50% of participants in our study did not consume red meat in the previous 24
h. We were not able to identify the factors explaining the high levels of PAH in all participants, suggesting that there is no single factor responsible for this pattern in our study population and, very likely, in Golestan. The potential differences in pattern of exposure in summer and winter also points to variability of major sources of PAHs. Results of this pilot study may be helpful in determining the issues to be focused on in future studies, e.g., PAH content in fried foods and potential seasonal variation in sources of PAHs. Further studies on biomarkers of internal exposure (e.g., PAH-related DNA adducts) and on the potential association of exposure to PAHs and risk of esophageal cancer are also warranted. Due to possible associations of exposure to ingested PAHs with several other health outcomes, including cancer of the urinary bladder and cardiovascular diseases (Curfs et al., 2005
; Ramos and Moorthy, 2005
; Baan et al., 2009
), the benefits of identifying and avoiding the preventable exposures to PAHs in the general population may go beyond esophageal cancer.