Chronic toxicity refers to health problems that may arise from repeated or prolonged exposure to smaller doses of pesticide. Some pesticides are carcinogenic, and some have the potential to affect nervous, hormonal or immune systems. There are many studies showing that chronic exposure to pesticides may increase the risk of a wide range of serious health problems, including certain cancers, neurological problems such as Parkinson’s disease, diabetes, respiratory diseases, some birth defects such as hypospadias, and reproductive problems such as reduced sperm count.
The risks will vary around the world according to local pesticide use and practices. Exposure is likely to be greatest for those using pesticides occupationally. Problems are arguably likely to be more serious in developing countries, for the same reasons that acute poisoning is more prevalent; however most of the research into chronic toxicity takes place in more industrialised countries.
Although there is a great deal of research linking pesticide exposure with a wide range of health problems, establishing the relationship between pesticide exposure, and a disease which may not manifest until many years later, is extremely problematic. Over their lifetimes, people are exposed to a huge range of naturally occurring and synthetic substances, so exposure patterns are very complex, and quantifying in retrospect the degree of exposure someone has had to a particular pesticide is very difficult. The diseases under consideration are often described as ‘multifactorial’ meaning that multiple environmental, lifestyle and genetic factors interact to determine each individual’s risk of developing that disease. Some of our knowledge of pesticide health effects comes from animal or laboratory studies, but it is difficult to extrapolate from these into what would happen in a human population. All this means that in many cases we simply don’t know what the long term effects of exposure to a pesticide, or to mixtures of pesticides, may be.