Pesticides are only effective for a certain period of time. Their shelf life depends on their active ingredients and the type of formulation.

If not stated otherwise on the label, products normally have a shelf life of two years from the date of manufacture. During this period the manufacturer guarantees the quality of the product, provided that it is stored according to instructions stated on the label.

It's not always easy to establish whether old pesticide stocks have deteriorated to such an extent as to become unusable. Storage periods beyond two years, or beyond the shelf life indicated on the label, do not automatically mean that the products are unusable. Pesticides can often be stored for much longer than their indicated shelf life. However, it may also happen that improper storage will accelerate deterioration to such an extent that the product becomes unusable well before its expiry date.

Often it's obvious that a pesticide whose expiry date has passed has become obsolete. Clear liquid formulations may have formed flakes or crystals. Emulsions and powders may have solidified. More difficult to identify are products whose chemical properties have changed, while the visible physical properties remain the same. In such cases, it is necessary to conduct chemical analysis in a laboratory to establish whether the product is still usable. Until such an analysis is done, the product is considered obsolete.

Some pesticides may be effective, but cannot be used because of bans placed on them for environmental and public health reasons. These pesticides are also obsolete.

Some stockpiles contain products that are still usable but are not wanted. There can be a number of reasons for this, such as excess stock, a greatly reduced pest problem, logistical constraints concerning distribution and product formulations that are not suitable for the application equipment.

Unwanted pesticides may not be obsolete. However, stocks that are not being used, run a high risk of becoming obsolete as a result of prolonged storage.

For more details about the causes of pesticide obsolescence, please visit the "Why do we have this problem?" section.

The WHO estimates that 3 million people are poisoned by pesticides every year, most of them in developing countries. Every year some 20 000 of these poisoning victims die.

Pesticides can be absorbed by contact with the skin, inhaled as dust or vapour or injested with contaminated food or water. Many people, children in particular, have been fatally poisoned by mistakenly drinking pesticides stored in bottles used for drinks.

When the pesticides have deteriorated and leaked from rusted containers or poured out of broken sacks, the threat of pesticide poisoning is very high, especially for children. There are many examples of children playing, livestock grazing, people working, cooking, drawing water and growing food around dumped and leaking pesticides.

When pesticides leak into the environment, chronic poisoning can effect entire communities. Symptoms of chronic poisoning include numbness or weakness of arms, legs feet or hands, lethargy and loss of memory and concentration and anxiety. Exposure to pesticides can affect the reproductive system, causing infertility, early pregnancy loss, spontaneous abortions and foetal deaths. Many pesticides are known to cause cancer. The impact on health and environment depends as much on the type of pesticide as the level of exposure.

Organochlorine pesticides contain carbon and chlorine atoms joined together. They are very toxic chemicals.

Organochlorines don't break down easily and can remain in the environment for a long time. They can evaporate in hot climates, travel through the atmosphere and settle in colder environments. As they move up through the food chain, they become more concentrated. The highest levels of organochlorines are found in human beings, fish-eating birds and marine mammals.

Because of their persistence, organochlorine chemicals became known as Persistent Organic Pollutants, or POPs. Many organochlorine pesticides, dieldrin in particular, were widely used in campaigns to eradicate locusts in Africa. However, as the world became more aware of the dangers of <acronym title="Persistent Organic Pollutants">POPs</acronym> chemicals, ogranochlorine pesticides were banned from use in locust campaigns.

The Stockholm Convention on Persistent Organic Pollutants, adopted in 2001, seeks to eliminate or severely restrict the production of POPS chemicals. The Convention specifically identified 12 chemicals for elimination. Of these, nine are pesticides: 

• aldrin
• chlordane
• endrin 
• dieldrin
• heptachlor
• DDT
• Toxaphene (Campheclor)
• Mirex
• hexachlorobenzene

For information about these pesticides click here or the links on the right.

When they were banned, these pesticides became instantly unusable. As a result, pesticides included in the Stockholm Convention are no longer being made or sold. DDT continues to be manufactured. Some countries have asked for and been granted an exemption to use DDT in anti-malaria campaigns. In such cases, DDT should only be used in accordance with WHO guidelines: http://www.who.int/malaria/docs/FAQonDDT.pdf . However, in countries where DDT is used in the health sector it is often found 'leaking' into other sectors such as agriculture where its use is not permitted.

Many other organochloride pesticides, with similar characteristics as the POP pesticides, continue to be sold and used. These include: 

• pentabromodiphenyl ether
• chlordecone
• hexabromobifenyl 
• lindane

The above pesticides have been banned in some countries and governments have officially proposed that they be reviewed for inclusion on POPs chemicals list.

The second generation of insecticides developed were based on organophosphates. These pesticides are much less persistent than the organochlorines but they are also much more toxic.

Byproducts of nerve agents developed during World War II, organophosphate pesticides are neurotoxins that can attack the nervous system. Exposure to organophosphates can cause dizziness, vomiting, seizures, paralysis, loss of mental function, and death. Almost half of all the WHO class 1a and 1b pesticides are organophosphates.

Many organophosphate pesticides have been banned or their use has been severely restricted in many countries. Several organophosphate pesticides are included the Rotterdam Convention's Prior Informed Consent (PIC) process. The Rotterdam Convention on the Prior Informed Consent Procedure for Certain Hazardous Chemicals and Pesticides which was adopted in 1998, is designed to help prevent the unwanted import of extremely hazardous pesticides and other chemicals to developing countries. The organophosphate pesticides currently included in PIC process are:

• Parathion (WHO class - Ia)
• Parathion methyl (WHO class - Ia)
• Phosphamidon (WHO class - Ib)
• Methamidophos (WHO class - Ib)

Organophosphate pesticides have a more limited shelf life than organochlorines and can become physically and chemically altered with time.

Synthetic pyrethroids are chemicals based on natural insecticides derived from chrysanthemum flowers. Developed as a less toxic alternative to organophosphates, only three pyrethroid pesticide formulations are classified as highly hazardous by the WHO.

In industrialized countries, synthetic pyrethroids are now often used instead of organophosphate pesticides. However, they are more expensive, and buyers in developing countries often prefer to purchase the cheaper, but more hazardous organophosphates or organochlorines that remain in use.