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Food Safety Focus ( 61st Issue, August 2011 ) – Food Safety Platform

Obsolete Pesticides – What Is Wrong?

Reported by Dr. John LUM, Scientific Officer,
Risk Assessment Section, Centre for Food Safety

The development of pesticides is an on-going process and new pesticides are developed by scientists to replace old ones. What is wrong with these old pesticides and why are they stopped from use?

Why Do Pesticides Become Obsolete?

When new pesticides that are cheaper with a relatively better efficiency or efficacy are available on the market, they will gain popularity and their counterparts will be phased out gradually. On the other hand, the international authority may restrict or even prohibit the use of certain pesticides due to their acute or long-term toxicity to human as well as their risks to the environment. Let us look at some examples in the following paragraphs.

Pesticides with Acute Toxicity Concern

The World Health Organization (WHO) classifies pesticides by hazard based on their acute risk to human health. Each pesticide is put into one of five classes, as published in the WHO recommended classification of pesticides by hazard :

Ia: Extremely hazardous
Ib: Highly hazardous
II: Moderately hazardous
III: Slightly hazardous
U: Unlikely to present acute hazard

Those that are classified as Ia or Ib are the most hazardous and capable of killing experimental animals at a relatively lower dose of exposure (e.g. those Ia pesticides could kill half of a group of test animals with less than 5mg/kg body weight). Many organophosphate (OP) insecticide are Class Ia substances and their effects on mammalian nervous systems are of particular concerns. In fact, almost half of all the pesticides classified by WHO as " Extremely hazardous" are organophosphates. Use of organophosphate insecticides has been severely restricted or even banned in many countries. A less toxic alternative to organophosphates in use is synthetic pyrethroids, which are chemicals based on natural insecticides derived from chrysanthemum flowers.

Pesticides with Undesirable Long-term Effects on Human Health

Some pesticides are not acutely toxic to human but have more concerned undesirable long-term effects, especially cancer causing or affecting the reproductive systems. For example, nitrofen is a herbicide with low acute oral toxicity. However, when tested through oral exposure in animals, it has been shown to affect developing foetus (i.e., teratogenic) and cause cancer in adult ones. As such, the use of nitrofen has been prohibited or phased out in many countries, including the Mainland, the USA and Canada, and replaced by newer and safer herbicides such as glyphosate.

Pesticides with Unacceptable Risks to the Environment

Some pesticides may deposit in the environment, such as water and soil, and subsequently affect animal and plant growth, and may eventually enter the food chain at various levels of the ecosystems. Therefore, hazardous pesticides or other toxic chemicals that may persist in the environment for a prolonged period will be unacceptable for intentional use. Many organochlorine (OC) insecticides, once popular throughout the world, have been removed from agricultural use for the above reasons. DDT is one of the best known examples. It is highly toxic to aquatic animals and its metabolite decreases the reproductive rate of birds by causing eggshell thinning and embryo deaths. The agricultural use of DDT has been banned in many countries. In fact, DDT and some OC pesticides (such as aldrin, dieldrin, chlordane and lindane) have been listed in The Stockholm Convention on Persistent Organic Pollutants. The Convention is administered by the United Nations Environment Programme which requires the parties to take measures to eliminate or reduce the release of these chemicals into the environment.

Proper Use of the Double-edged Sword

In summary, the agricultural use of pesticides is a double-edged sword – their proper use could enhance crops production while minimising the harmful effects to human and the environment. When pesticides are used for crop protection, they must be handled safely and responsibly. Suitable pesticides should be selected for crops and pests concerned and only registered pesticides which are properly labelled and packaged should be purchased. Safety guidelines on the use and handling of pesticides of regulatory authorities should be observed.