Exposure intervention and health impact assessment : The example of baker's asthma

Abstract

The regular occurrence of respiratory allergies associated with occupational exposure to flour dust and other allergens, initiated a covenant supported by major flour processing sectors (bakeries, flour mills and ingredient producers) and social partners. The main goal was to decrease occupational exposures and associated disease burden. Exposure surveys were performed to assess exposure levels to flour dust and allergens and enable evaluation of the impact of the covenant actions. A health surveillance program was initiated to monitor the health status of the working population and intervene when an individual was likely to have a respiratory allergy. To evaluate the impact of potential exposure reductions on the population’s health, an assessment methodology was needed. Consequently, a quantitative health impact assessment methodology was developed. The dynamic population-based model was used to assess the impact of different intervention strategies on the disease burden of work related respiratory symptoms in Dutch bakery workers. The dynamic population-based model was developed using information from the exposure surveys and epidemiological studies performed during the covenant. The model simulates a population of individual workers longitudinally and tracks the development of work-related sensitisation, respiratory symptoms and work disability in each worker. Each worker’s disease state is modelled independently using a discrete time Markov Chain, updated yearly using each individual’s simulated exposure. The model provides a valuable population level representation of the mechanisms contributing to respiratory diseases in bakers, and can be used for quantitative health impact assessment. This is demonstrated through the evaluation of the impact of different intervention strategies on the disease burden of the population over time. Different interventions based on 1) exposure reductions for wheat and fungal alpha-amylase allergens 2) health surveillance combined with individual reduction of exposure for high risk workers and 3) pre-employment screening, were evaluated. The impact of the interventions was compared with the baseline intervention program performed as part of the covenant. The predicted impact of the covenant was modest with a change in the prevalence of the different disease states of less then -15% after 20 years. The impact of most of the other intervention strategies evaluated was higher, but generally less then -50% for lower respiratory symptoms and work disabling asthma. Only the rigorous health surveillance intervention scenario, in which workers are identified who are sensitized or report upper respiratory symptoms and for whom the exposure is decreased by 90%, resulted in a predicted decrease of almost 60% in disease burden after 20 years. These results give an indication of the most effective intervention strategy for decreasing the occupational respiratory disease burden in bakery workers. It also provides more generic lessons for intervention research. This information can assist policy makers in their choice of intervention strategy and gives an indication of achievable reductions in disease burden over time. The different studies in this thesis illustrate the importance and complexity of creating a good quantitative evidence base for occupational intervention. In many occasions, like in many of the Dutch covenants, the evidence base for effective interventions remains weak and can be optimized using quantitative health impact assessment methods.