The Photokinetics and Environmental Fate of Indoor Pyrethroid Pesticides Implications for Long Term Accumulation and Human Exposure, and Steps Towards Knowledge Translation for Public Health

Abstract

Humans spend approximately 90% of their time indoors, where they are exposed to a ‘cocktail’ of pollutants from both natural and human-influenced sources. These pollutants undergo a plethora of complex transformational processes that include photochemistry. One important group of indoor pollutants are pyrethroid pesticides, which are a popular measure for the control of malarial vectors and household insects. In this thesis, my primary goal was to understand and bring to light the currently understudied indoor chemistry of pyrethroid pesticides using a ‘three-legged approach’ and discuss potential implications to human exposure and knowledge translation. The first project investigated the photochemistry of pyrethroid formulations in the laboratory, under indoor relevant conditions. Pyrethroid photochemistry has previously been studied from an outdoor context especially post-agricultural application, using commercial standards as samples. However, indoor photochemistry is unique due to low light intensities and long wavelengths of light present, and indoor pesticides mainly exist as formulations (i.e., a mixture of pesticides and non-pesticidal ingredients). In this project, I show that pyrethroid pesticides photolyze under indoor illumination, and that their photodegradation is influenced by their chemical environment (formulation versus pure standard), environmental conditions prevailing indoors, and human activities (e.g., cooking). The second project was a follow up on the first project, where I investigated the persistence of pyrethroid pesticides in different rooms of an occupied residential home. Here, I show that pyrethroids can persist on surfaces in all rooms except in areas that are directly exposed to sunlight; in addition, and for the first time, I demonstrated that their persistence can be enhanced by human activities such as cooking. These findings suggest that pyrethroids persist indoors for lengthier periods compared to outdoor environments. In the third project, I designed and conducted a pilot study in Harare, Zimbabwe, aimed at investigating the potential accumulation and residual footprint of pyrethroid pesticides in residential homes, and I sought to understand what their presence could mean in terms of human exposure via dermal and non-dietary routes. I show that this class of compounds is ubiquitous in residential households, and they can accumulate over time due to continuous use. I also find evidence of other pesticides that are not disclosed on pesticide product labels, some of which are categorised as carcinogens. Overall, this work provided previously unknown knowledge on the chemistry of these pesticides indoors, and its potential implications on human exposure and future knowledge translation work that will be useful to inform public health policy makers and consumer product regulators.