Researchers from the Oregon Health & Science University, USA have

demonstrated that environmentally relevant concentrations of organophosphate pesticides (OP) increased the sensitization to allergens.

They concluded: ‘Because sensitization to allergens is characteristic of 50% of the general population and 80% of asthmatics (including children), these findings have significant implications for OP risk assessment, intervention, and treatment strategies.’ Source: Environmental Health Perspectives

An earlier study published in the American Journal of Respiratory and Critical Care Medicine found 'an increase of 50 percent in the prevalence of allergic asthma in all farm women who applied or mixed pesticides.' Parathion was associated with a three-fold increase in allergic asthma while malathion exposure was linked to a 60% increase.