Common environmental chemicals do not explain atopy contrast in the Finnish and Russian Karelia

The study areas and sampling methods have been described in detail earlier [1]. In brief, the study was carried out in North Karelia in eastern Finland and about 180 km away in Pitkäranta Region in the Republic of Karelia in northwestern Russia. Atopy data were obtained by using a self-administered questionnaire and skin prick tests (SPT). SPT was performed using a standard set of nine common inhalant and five food allergens. The subject was considered atopic, if he or she had at least one SPT response with a wheal diameter of 3 mm or larger and the positive and negative controls gave expected results [7]. Originally, 546 child–mother pairs from Finland and 550 from Russia were enrolled in the study. The data for the present analysis was obtained by taking a random subsample of 200 subjects: 25 atopic (SPT positive) and 25 non-atopic (SPT negative) children and their mothers in Finland and Russia. Out of the mothers 20 were atopic in Finland and 23 in Russia. The children were aged 6–15 years (mean 11.2) and mothers 27 to 50 years (mean 37.5). Data on ?-HCH were available only for 30 children and their mothers in Finland and Russia and of them 15 children and 11 mothers were atopic in Finland and 20 children and 13 mothers were atopic in Russia.

A total of six polychlorinated biphenyls (PCBs congener numbers 118, 138, 153, 156, 170 and 180), 1,1-Bis-(4-Chlorophenyl)-2,2,2-trichloroethane (DDT),

1,1-Dichloro-2,2-bis-(p-chlorophenyl)-ethylene (DDE), hexachlorobenzene (HCB), ?-hexachlorocyclohexane (?-HCH), and 2,2?,4,4?-tetra-bromodiphenyl ether (BDE47) were measured in 200 µl aliquots of sera from children and mothers. Details of sample pretreatment for the persistent organic pollutants have been published recently [8]. In each batch of samples (n = 25), two blanks were included to control for possible laboratory background or cross sample contamination. Additionally, two control serum samples were added to assess the between-assay coefficient of variation which varied between 2.1 and 4.0 %. Average recoveries of measured POPs in control samples were 97–106 % of the certified values. The limits of quantitation (LOQs) for the PCBs, DDE, HCB, ?-HCH, and BDE47 were between 2 and 5 pg/mL. The LOQ for DDT was 20 pg/mL. Values below LOQ were measured in the Finnish children as follows N (%): PCB118 6 (12 %), PCB156 22 (44 %), PCB170 4 (8 %), DDT 50 (100 %) and in the Finnish mothers HCH 8 (27 %), DDT 50 (100 %), BDE47 37 (74 %) and PCB156 3 (6 %). The respective values for Russian children were: DDT 17 (34 %) and BDE47 48 (96 %) and for the Russian mothers DDT 16 (32 %) and BDE47 47 (94 %).

In the results we used the raw data of analyses employing the best estimate of the concentration of an analyte, even below the LOQ, in order to avoid extensive amount of LOQs in statistical estimations. The analyses were performed at the National Institute for Health and Welfare, Chemicals and Health Unit, Kuopio, Finland. The Unit is an accredited testing laboratory (T077: ISO/IEC 17025) by Finnish Accreditation Services (FINAS).

Chemical concentrations are reported as unadjusted means with confidence intervals. The distributions of the measured chemicals were skewed so the levels of the POPs were log2-transformed. Correlations between log2-transformed concentrations were tested with Pearson correlations. Mann–Whitney U-test and logistic regression were used for the statistical analyses and p value below 0.05 was considered significant. SPSS version 22 was used for the statistical analyses.

The study protocol was approved by the Coordinating Ethics Committee of the Helsinki University Hospital District. All participants gave their informed consent.