"Sexual maturation in relation to polychlorinated aromatic hydrocarbons ...": Den Hond et al.'s response. (Correspondence).

The Flemish Government commissioned the Environment and Health Study after a competitive call for research proposals in 1998 (Onderzoek Milieu en Gezondheid 1998). From the outset, this research project included two distinct population-based surveys, one in adolescents (16-18 years of age) and another in adults (21-65 years of age). We assume scientific responsibility only for the project in adolescents.

Our study (which was carried out in 1999) sought to investigate whether, in adolescents, biomarkers can show exposure to and health effects of common environmental pollutants (Staessen et al. 2001). We planned to maximize the gradient in environmental exposure by recruitment of adolescents from a rural control area (Peer, Belgium) and a polluted industrial suburb. For logistic reasons, the adolescents were recruited from and were examined at local schools. Life-long residence in the study areas was a prerequisite for participation. The Flemish Government imposed the selection of the polluted area because of a long-lasting controversy about the possible impacts on public health of two waste incinerators in Wilrijk and a large nonferrous smelter in Hoboken. Health authorities assumed that our study would settle the local concern about detrimental health outcomes associated with the waste incinerators in Wilrijk. In spite of the imposed selection of the polluted area, our study had the advantage of a clear-cut contrast in exposure between the control and polluted areas, as evidenced by all available records on environmental monitoring.

Molenberghs et al. found the lack of randomness a major shortcoming of our study design. However, they miscalculated the proportion of boys to girls, which was 1.5 (60/40) in Peer, 1.0 (21/21) in Wilrijk, and 2.1 (39/19) in Hoboken. We believe that, in the context of the imposed selection, we maximally exploited the remaining degrees of freedom to avoid serious selection bias. Indeed, participants and non-participants had similar age, sex distribution, parental social class, and regional residence. In the industrial suburb they resided at similar distances from the main sources of pollution, which strongly suggests random selection with regard to exposure (Staessen et al. 2001). Although we cannot extrapolate to the general Flemish population, our conclusions are applicable to the adolescents enrolled in our population-based sample.

After our Data Monitoring Committee--which was enlarged by three international experts for peer-review--had discussed the study outcomes on 12 May 2000, our results became available for disclosure to the public and subsequent scientific publication. The health administration then appointed Molenberghs and colleagues as the official external reviewers of our study (Goetghebeur et al. 2000; Cuijpers et al. 2000). These statisticians seem to imply that our conclusions should have been restricted to the feasibility of biomonitoring in adolescents and that we should not have disclosed the unexpected medical findings suggesting relevant environmental health risks. To what extent their criticisms are justified and independent remains open for discussion. In this context, it is interesting to note that, on 25 July 2000, the Flemish Minister of Public Health addressed a letter to the principal investigator of our study (J.A.S.) demanding that disclosure of the findings be postponed for 6 months until the study in adults would be completed. This embargo was lifted on 21 November 2000, a few weeks after the municipal elections.

In contrast to the criticism of Molenberghs et al., we actually based our study on a priori hypotheses associating defined biomarkers of exposure with relevant effect markers (Staessen et al. 2001). This is also the case for the specific article on the endocrine effects of polychlorinated aromatic hydrocarbons (PCAHs) (Den Hond et al. 2002) and other recent papers that emanated from the adolescent study, for example, those on the immune (Van Den Heuvel et al. 2002) and genotoxic (Verheyen et al. Unpublished data) effects. Because we were concerned about the reproducibility of the staging of sexual maturation according to Marshall and Tanner's scale (Marshall and Tanner 1969; Marshall and Tanner 1970) and the measurement of testicular volume using Prader's orchidometer (Dornberger et al. 1987), we mounted a separate validation study of which the results have been reported and discussed elsewhere (Den Hond et al. 2001).

Molenberghs et al. confuse medically assisted conceptions with medically assisted deliveries. Nevertheless, we acknowledge that age is a major determinant of fertility in women. However, the officially published report did not include an age-adjusted analysis of the proportion of medically assisted conceptions (Advoet et al. 1999). The median age in women at first delivery was 27.3 (interquartile range 25.2-29.3) years in Wilrijk and 26.6 (24.2-29.0) years in Flanders. A difference of 0.7 years cannot explain a 65-75% increase in medically assisted conceptions (5.6% in Wilrijk vs. 3.4% in Flanders for singleton births and 59.0% vs. 33.4% for multiple births).

We hope that, with these clarifications, our study is more clearly placed in the perspective of the Flemish Environment and Health Study. We agree with Molenberghs et al. that our findings, although in line with the concept of endocrine disruption and Sharpe and Skakkebaek's hypothesis, require further research. As in all cross-sectional epidemiologic studies, we did not prove causation. However, numerous experimental studies support the causal association between exposure to PCAHs and effects on sexual maturation.

REFERENCES

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Cuijpers C, Molenberghs G, Passchier W, Pieters J. 2000. De vinger aan de pols van een gezond Vlaams milieu (2) (Dutch). Brussels:Ministerie van de Vlaamse Gemeenschap. Available: http://www.wvc.vlaanderen.be/ gezondmilieu/onderzoeken/koepel/rapport_review-commissie/pdf/eindrapport.PDF [cited 25 October 2002].

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Den Hond E, Roels HA, Hoppenbrouwers K, Nawrot T, Thijs L, Vandermeulen C, et al. 2002. Sexual maturation in relation to polychlorinated aromatic hydrocarbons: Sharpe and Skakkebaek's hypothesis revisited. Environ Health Perspect 110:771-776.

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Goetghebeur E, Passchier W, Pieters J. 2000. De vinger aan de pols van een gezond Vlaams milieu. Brussels:Ministerie van de Vlaamse Gemeenschap (Dutch). Available: http://www.wvc.vlaanderen.be/gezondmilieu/onderzoeken /koepel/toxicologie/pdf/addenda_adolescenten /12/pr000607.PDF [cited 25 October 2002].

Marshall WA, Tanner JM. Variations in the pattern of pubertal changes in girls. Arch Dis Child 1969; 44:291-303.

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Onderzoek Milieu en Gezondheid. 1998. Ontwikkeling van een gestructureerde aanpak voor vermoedelijke aan het milieu gerelateerde ziekteclusters in Vlaanderen en van een leefmilieu- en epidemiologische monitoring (Dutch), Brussels:Ministerie van de Vlaamse Gemeenschap.

Staessen JA, Nawrot T, Den Hond E, Thijs L, Fagard R, Hoppenbrouwers K, et al. 2001. Renal function, cytogenetic measurements and sexual development in adolescents in relation to environmental pollutants: a feasibility study of biomarkers. Lancet 357:1660-1669.

Van Den Heuvel RL, Koppen G, Staessen JA, Den Hond E, Verheyen G, Nawrot TS. 2002: Immunologic biomarkers in relation to exposure markers of PCBs and dioxins in Flemish adolescents (Belgium). Environ Health Perspect 110:595-600.

Elly Den Hond
Hypertensie en Cardiovasculaire
Revalidatie Eenheid
Departement Moleculair en
Cardiovasculair Onderzoek
Leuven, Belgium
E-mail: [e-mail]

Harry Roels
Unite de Toxicologie Industrielle et de
Medicine du Travail
Universite catholique de Louvain
Bruxelles, Belgium

Jan A. Staessen
Hypertensie en Cardiovasculaire
Revalidatie Eenheid
Departement Moleculair en
Cardiovasculair Onderzoek
Leuven, Belgium