Exposure to multiple pesticides and the risk of non-Hodgkin lymphoma

Hohenadel K, Harris SA, McLaughlin JR, Spinelli JJ, Pahwa P, Dosman JA, Demers PA, Blair A. Exposure to multiple pesticides and risk of non-Hodgkin lymphoma in men from six Canadian provinces. International Journal of Environmental Research and Public Health 2011; 8(6):2320-2330.

ABSTRACT: Non-Hodgkin lymphoma (NHL) has been linked to several agricultural exposures, including some commonly used pesticides. Although there is a significant body of literature examining the effects of exposure to individual pesticides on NHL, the impact of exposure to multiple pesticides or specific pesticide combinations has not been explored in depth. Data from a six-province Canadian case-control study conducted between 1991 and 1994 were analyzed to investigate the relationship between NHL, the total number of pesticides used and some common pesticide combinations. Cases (n = 513) were identified through hospital records and provincial cancer registries and controls (n = 1,506), frequency matched to cases by age and province of residence, were obtained through provincial health records, telephone listings, or voter lists. In multiple logistic regression analyses, risk of NHL increased with the number of pesticides used. Similar results were obtained in analyses restricted to herbicides, insecticides and several pesticide classes. Odds ratios increased further when only ‘potentially carcinogenic’ pesticides were considered (OR[one pesticide] = 1.30, 95% CI = 0.90–1.88; OR[two to four] = 1.54, CI = 1.11–2.12; OR[five or more] = 1.94, CI = 1.17–3.23). Elevated risks were also found among those reporting use of malathion in combination with several other pesticides. These analyses support and extend previous findings that the risk of NHL increases with the number of pesticides used and some pesticide combinations.

What is non-Hodgkin lymphoma?

Non-Hodgkin lymphoma (NHL) is a term used to describe a group of blood cancers that start growing in white blood cells, which are part of the body’s immune system. NHL occurs in the lymph nodes, spleen, thymus and bone marrow. The causes are unclear, but NHL has been linked to autoimmune diseases, a weakened immune system, nuclear radiation and exposure to certain chemicals including pesticides.

What is the effect of multiple pesticide exposure on the risk of non-Hodgkin lymphoma?

Exposure to individual pesticides has been linked to an increased risk of developing NHL, but the impact of multiple pesticides is harder to establish. The Occupational Cancer Research Centre aims to fill in the knowledge gaps in occupational cancer research; to use what is currently known in order to minimize workplace carcinogenic exposures and, ultimately, improve workers’ health. Researchers at the OCRC have recently completed a study on the effect of the exposure to multiple pesticides on the risk of NHL. This study focused on Canadian men ages 19 and older. Data was collected between 1991 and 1994 as part of the Cross-Canada Study of Pesticides and Health. Details of the men’s medical and occupational history, age and other demographic details, and exposure to pesticides were collected by postal questionnaires and telephone.

Pesticides should be treated with caution

The OCRC found that the risk of developing NHL increases with the number of pesticides employed. This is especially notable when the analysis is limited to potentially carcinogenic pesticides. Exposure to one potentially carcinogenic pesticide means a 1.3 times greater risk of developing NHL. Exposure to two to four pesticides increases the risk by a factor of 1.5. Exposure to five or more pesticides doubles the risk of NHL.

Why does the use of multiple pesticides lead to a greater risk? One possible explanation is that, as the number of pesticides increases, the chance of encountering one or more pesticides with significant carcinogenic properties similarly increases.  Alternatively, it may also be that some combination of pesticides together increases the risk of cancer.

While more research is needed to fully understand this phenomenon, it is clear that pesticides should be treated with caution—especially those that have been classified as potentially carcinogenic. It is always good practice to use the least toxic pesticides possible, including those that have less evidence of carcinogenicity, and to wear appropriate protective equipment to help to lower the risk of exposure.