The present study evidenced associations between childhood acute leukaemia and three sources of exposure to insecticides: home insecticide use, garden insecticide use, and insecticide use for pediculosis. The number of parents who were occupationally exposed to pesticides was too small to allow further analyses.
The size of the present study enabled detection of minimum odds ratios of 1.6, 1.9, and 2.2 for control exposure prevalences of 20%, 10%, and 5%, respectively. These prevalences are of the same order of magnitude as those for home insecticide use during pregnancy and during childhood (21% and 29%, respectively), garden insecticide use during pregnancy and during childhood (1% and 5%, respectively), and insecticidal pediculosis treatment during childhood (25%).
The oncology departments recruit patients from more distant places than do control departments, and this could have introduced bias. In order to keep cases and controls comparable in terms of socioeconomic category and rural/urban status, most of the children (all but seven cases and four controls) were living in the same administrative region as the hospital location.
The case and control mothers were very similar with respect to education, occupation, socioeconomic status, and place of residence. The results were unchanged after additional adjustment for the parents' socioprofessional categories, educational levels, place of residence (urban or rural), or type of housing (apartment or house). The use of standardised questionnaires and similar interviewing conditions for case and control mothers reduced potential differential misclassifications.
Pesticide exposure is a growing public concern which might induce recall bias. However, our study took place in the period 1995–99 when the subject was far less in the media in France than it is now. Nevertheless, a recall bias cannot strictly be ruled out. The information on shampoos to treat pediculosis may be unreliable, but probably in the same way for the cases and controls.
A variety of possible confounding factors were incorporated in the model in order to test the consistency of the association between insecticide exposure and acute leukaemia. The factors included variables that had previously been shown to be related to childhood acute leukaemia in the present study. The variables related to home or garden insecticide use and pediculosis treatment were also incorporated simultaneously. Adjusted for separately or taken together, none of those variables had any influence on the results. With respect to shampoos for pediculosis, sensitivity analyses showed that loss of association would only occur for unlikely distributions of missing data and that the OR would still be 1.2 in the extreme scenario, in which all the missing case data consisted of non‐exposures and all the missing control data consisted of exposures.
The shampoos used to treat pediculosis could have contained three types of insecticide, possibly in combination: pyrethroid, organochlorinated (lindane), and organophosphorus (malathion) insecticides. To the authors' knowledge, no previous study has investigated direct childhood pesticide exposure due to insecticidal shampoos. The results reported herein therefore need to be replicated and investigated further.
The results for residential pesticide exposure are consistent with previously published studies. Home pesticide use during pregnancy or childhood was associated with childhood acute leukaemia in the six studies which investigated that exposure.2,3,4,5,6,8
Leiss and Savitz (1995) reported an association with pesticide strip use during pregnancy and childhood.4
The authors cited dichlorvos, a specific insecticide used in pesticidal strips, which is carcinogenic in animals and classed as possibly carcinogenic for humans by the IARC. In addition, “spraying and application of non‐arsenical insecticides entailing exposures” have been classified as probably carcinogenic by the IARC.11
The association with garden pesticide use is less consistent: two studies found an association between childhood leukaemia and garden pesticide use during pregnancy,3,5
one study found an association with garden pesticide use during childhood,3
and three studies did not find any association irrespective of the period of exposure.4,6,8
The incidence of childhood cancer was not related to local agricultural pesticide use in an ecological study.17
However, the same authors subsequently conducted a case control study and reported associations between childhood leukaemia and local agricultural use of two common types of pesticide, metham sodium (OR
2.05 (95% CI 1.01 to 4.17)), and difocol (OR
1.83 (95% CI 1.0 to 3.22)).18
In conclusion, the findings of the present study reinforce the hypothesis already suggested by the literature that household pesticide exposure may play a role in the aetiology of childhood acute leukaemia. At this stage, no specific product can be singled out and a causal relation remains questionable. However, the consistency of our results and the results from previous studies suggests that it may be opportune to consider preventive action.