Research Article: An Urgent Need to Restrict Access to Pesticides Based on Human Lethality

Date Published: October 26, 2010

Publisher: Public Library of Science

Author(s): Matthew Miller, Kavi Bhalla

Abstract: Matthew Miller and Kavi Bhalla discuss new research findings from Andrew Dawson and colleagues on the human toxicity of pesticides in Sri Lanka, and call for urgent reclassification of agricultural pesticides to help reduce suicides by poisonings.

Partial Text: Agricultural pesticides account for at least 250,000 suicide deaths each year, making pesticides the single most common means of suicide worldwide [1],[2]. The proportion of suicide deaths attributable to pesticide self-poisoning varies considerably across the world: in Europe and the Americas fewer than 5% of suicide deaths involve pesticides; in the Eastern Mediterranean, African, and Southeast Asian regions approximately 20%–25% involve pesticides; and in the Western Pacific region more than half of all suicides are pesticide related [1]. In aggregate, pesticide poisoning is involved in one-third of all suicides. In Sri Lanka, the site of the study by Andrew Dawson and colleagues published in this week’s PLoS Medicine[3], more than half of all suicide deaths in 2005 were due to pesticides [4].

Over the past two decades, a series of targeted legislative initiatives in Sri Lanka culminated in the withdrawal of World Health Organization (WHO) class I pesticides and, eventually, endosulfan, resulting in a fall in the incidence of suicide by 50%. This decline in suicide by pesticides occurred without a compensatory increase in suicide by other methods [5]. Other notable examples of large-scale population level reductions in suicide incidence due to decreased access to and/or availability of highly lethal and commonly used suicide methods (an approach known as means restriction) have occurred in Samoa and the United Kingdom. In Western Samoa the rise and fall of suicides, but not suicide attempts, closely tracked the introduction and later banning of paraquat on the island [6]. In the UK during the 1960s, a shift in the source of household heating away from coal-gas (capable of producing lethal levels of carbon monoxide) to detoxified natural gas was followed by a 30% decline in national suicide rates. As in Sri Lanka, the dramatic decline in overall suicide incidence in the UK was driven by a decline in method-specific suicides (i.e., in carbon monoxide suicides) without compensatory increases in suicide by other (i.e., non-carbon monoxide related) methods. This natural experiment in the UK is known as the “Coal Gas Story” [7],[8], and is perhaps the most famous example of means restriction at the population level.

The study on pesticide lethality by Dawson and colleagues [3] adds an important observation to the evidence base for pesticide restrictions in South Asia. In the current study, data from two referral hospitals in Sri Lanka were used to calculate formulation-specific case fatality ratios for several pesticides used in acts of intentional self-harm. The authors found that the human toxicity of many pesticides commonly used in Sri Lanka today is sharply discordant with current WHO toxicity rankings (which are based predominantly on oral LD50 in rat models). Dawson and colleagues not only identify several pesticides commonly used in suicide attempts that are far more likely to prove lethal to humans than is suggested by current WHO toxicity classification (e.g., paraquat), they also identify readily available and significantly less toxic alternative formulations (e.g., glyphosate) with similar agricultural indications.

Findings from the current study by Dawson and colleagues suggest persuasively that human toxicity data should be incorporated into regulatory decisions. Integrating human toxicity data into the WHO toxicity schema has the potential not only to further reduce suicide rates in Sri Lanka, but also to dramatically reduce suicide incidence throughout the developing world—where hundreds of thousands of lives hang in the balance. As such, the WHO should immediately reclassify some formulations (e.g., dimethoate, fenthion, and paraquat) since the lethality of these compounds is several-fold higher than alternatives within the same chemical and functional class. For many other pesticides, more precise measures of human lethality are needed, highlighting the importance of sustainable funding streams to develop and maintain surveillance systems that collect data on intentional self-harm and for an iterative process whereby pesticides are periodically reclassified as new data become available.

Findings from the current study by Dawson and colleagues have helped refine human toxicity estimates for pesticides in use today. Better surveillance data and additional research will, eventually, lead to additional refinements. In the meantime, while we wait for these refinements, we must not ignore what, thanks to Dawson and colleagues, we already know. As Bradford Hill noted nearly half a century ago [11], the fact that all scientific knowledge is provisional does not, in his memorable words, confer upon us a freedom to ignore the knowledge we already have, or to postpone the action that it appears to demand at a given time.



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