Plagued by the thought of pests infesting their crops, farmers liberally spray their fields with insecticides. But although this keeps the fruits and vegetables we buy intact, in the process they often expose themselves to serious health risks.

A recent study by the Department of Neuroendochrinology and Experimental Haematology Unit of Chittaranjan National Cancer Institute (CNCI), Calcutta, and the WB government’s department of forest and environment reveals that agricultural workers spraying pesticides either by hand-held pumps or backpack tanks face the danger of being exposed to carcinogens.

Most insecticides, herbicides and fungicides used in agriculture and for domestic purposes belong to a class of pesticides called organophosphates (OP). The most common OP pesticides are malathion, chloropyrifos, dimetheoate and phosphamidon. These degrade fast and are fast-acting on the target pest. However, their toxicity is not specific to any particular pest, and lethal effects have often been observed in other, non-targeted organisms, especially birds and humans.

The study, conducted on 265 male agricultural workers, revealed that most farmers never took any safety measures while spraying the pesticides. “They do not wash their hands after spraying, least aware of the danger,” said Dr Twisha Lahiri, head of the Neuroendochrinology department, CNCI.

The farmers spray their fields for almost five hours a day in a particular season. With no protective gear in place, they are exposed to the harmful chemicals through inhalation and oral and dermal absorption.

OP pesticides trigger both leukaemia (cancer of the bone marrow) and lymphoma (cancer that originates in lymphocytes), said Dr M.R. Ray, head of the Experimental Haematology Unit. Lymphoma is broadly divided into two categories — Hodgkin’s and non-Hodgkin’s lymphoma. It has been observed that these pesticides trigger non-Hodgkin’s lymphoma on a larger scale, he said. Non-Hodgkin’s lymphoma poses a greater threat as it does not respond to treatments like chemotherapy and bone marrow transplant.

OP pesticides also affect the nervous system by inhibiting acetylcholinesterase (AChE) enzyme activity. This enzyme breaks down the neurotransmitter acetylcholine. Change in AChE’s molecular configuration makes it impossible for it to function properly, causing acetylcholine to accumulate. This increases nerve impulse transmission, thereby leading to nervous system failure. The respiratory muscles are the most affected muscle group, whose paralysis often causes death.

“Comparing blood samples of the exposed workers with that of those unexposed, we found certain haematological changes in the former group,” said lead researcher Sreeparna Chakrawarty. The count of white blood cells (WBCs) like neutrophils and lymphocytes decreased while that of monocytes and eosinophils increased. Reduction in WBCs makes the farmers more susceptible to infections, while the increase in monocytes and eosinophils indicates the vulnerability to allergies, explained Chakrawarty.

Moreover, though the platelet count remained the same both in the control group and the exposed workers, the platelets in the latter were activated. And the chances for such platelets to clump together are high, thereby increasing the risk of cardio-vascular diseases.

“Our study is aimed at educating policymakers about the occupational health hazards of the farmers,” said Lahiri. The report was recently presented at a national seminar organised by the University of Calcutta.

Bugs interrupted

Pesticides lower nitrogen-fixing capacity of soil bacteria. T.V. Jayan reports

Root nodules in a leguminous plant

Agrichemicals may pull the plug on one of the most prominent nutrient-building processes in nature: nitrogen fixation by soil microbes.

Years of research in the lab as well as on the farm has shown that artificial chemicals in pesticides sprayed on crops disrupt natural nitrogen-fixing communications between crops and soil bacteria. This disruption leads to lower yields and/or significantly delayed growth, according to a study appearing in the forthcoming issue of the Proceedings of the National Academy of Sciences.

Jennifer Fox, a postdoctoral researcher at the Center for Ecology and Evolutionary Biology, University of Oregon , and her colleagues demonstrated that certain agrichemicals bind and block connections to specific receptors inside rhizobia bacteria living in root nodules in the soil. Rotation legume crops such as alfalfa and soybeans require such interaction to naturally replace nitrogen levels that, in turn, benefit crops such as corn grown after legume plantations.

Legume plants secrete chemical signals that recruit the friendly bacteria, which work with the plants to convert atmospheric nitrogen into ammonia, which is then used as fertiliser by the plants.

“Agrichemicals are blocking the host plant’s phytochemical recruitment signal,” Fox said. “In essence, they are cutting the lines of communication between the host plant and symbiotic bacteria. This is the mechanism by which these chemicals reduce symbiosis and nitrogen fixation.”

The scientists tested more than 50 chemicals. Though none of the chemicals used in the research seemed toxic to either the plants or bacteria, nearly 20 of them inhibited nitrogen fixation, with varying amounts of signal disruption.

“Our research provides another explanation for declining crop yields,” Fox said. “We showed that by applying pesticides that interfere with symbiotic signalling, the overall amount of symbiotic nitrogen fixation is reduced. If this natural fertiliser source is not replaced by increased application of synthetic nitrogen fertiliser, then crop yields are reduced and/or more growing time is needed for these crops to reach the yields obtained by untreated crops. This is a previously unforeseen factor contributing to declining crop yields.”