Mosquito coil smoke inhalation toxicity. Part I: validation of test approach and acute inhalation toxicity.

Burning mosquito coils indoors to repel mosquitoes is a common practice in many households in tropical countries. The evaluation and assessment of the inhalation toxicity of smoke emitted from mosquito coils appear to be particularly challenging due to the complex nature of this type of exposure atmosphere. The potential health implications of the gases, volatile agents and particulate matter emitted from burning coils or incense have frequently been addressed; however, state-of-the-art inhalation toxicity studies are scarce. The focus of this paper was comparatively to evaluate and assess the appropriateness and practical constraints of the whole-body versus the nose-only mode of exposure for inhalation toxicity studies with burning mosquito coils. With regard to the controlled exposure of laboratory animals to complex smoke atmospheres the nose-only mode of exposure had distinct advantages over the whole-body exposure, which included a rapid attainment of the inhalation chamber steady state, minimization of particle coagulation and uncontrolled adsorption of condensate onto the chamber surfaces. While in whole-body chambers a different kinetic behaviour of volatile and particulate constituents was found which caused inhomogeneous, i.e. artificially enriched atmospheres with volatile components at the expense of aerosols, the nose-only mode of exposure provided maximum exposure intensities without losses of the particulate phase of the exposure atmosphere. Collectively, the results obtained support the conclusion that the dynamic nose-only mode of exposure is experimentally superior to the quasistatic whole-body exposure mode which provides the least control over exposure atmospheres and the outcome highly contingent on selected experimental factors. Acute inhalation toxicity studies in rats suggest that the most critical metrics of exposure are apparently related to (semi)volatile upper respiratory tract sensory irritants, whilst the asphyxic component, carbon monoxide, plays a role only at overtly irritant exposure levels. However, this study was conducted at exposure concentrations much higher than encountered by people in residential settings and the effects observed under these conditions may not be relevant to hazards from exposures at common use levels. Neither an acute 8 h exposure of rats nor the 1 h sensory irritation study in mice and rats provided experimental evidence that irritant particle-related effects had occurred in the lower respiratory tract. In summary, the protocols devised evaluate and assess the acute inhalation toxicity of mosquito coil smoke demonstrating that the nose-only mode of exposure of rats to the smoke of mosquito coils is suitable to assess the toxic potency of different coils. The nose-only mode has clear advantages over the whole-body exposure mode. The inhalation studies conducted show unequivocally that acute toxic effects are difficult to produce with this type of product even under rigorous testing conditions.