Pesticide classes and associated hazards

All pesticides contain active ingredients that control pests by interfering with their natural body functions.

The body functions that are interfered with may be similar in non-target species – including humans – and therefore may present a hazard to health.

The chemical structure of the active ingredient determines what chemical class that particular pesticide belongs to. The class is not necessarily written on the pesticide label or the Material safety data sheet (MSDS). However, it is useful to understand the properties and modes of action of the most common chemical classes to:

• use these pesticides effectively
• appropriately advise clients
• minimise potentially adverse health effects.

A chemical class may also be referred to as a chemical family or chemical group.

Do not confuse chemical classes with the authorisations listed on the back of your licence to use a pesticide. The licence authorisations depend on the type of pest control work that you are qualified to undertake, not the chemical class of the pesticide that you are permitted to apply.

Pesticide classes – summary

Table 1 summarises the classes of pesticides most commonly used; more detailed information is available on subsequent sections of this page.

Table 1: Summary of pesticide class and their characteristics

Notes: Acetylocholinesterase inhibitors and sodium channel modulators affect the nervous system.

Pesticide precautions – summary

When using pesticides, remember to:

• read the label instructions and follow them carefully
• consult the MSDS for safety instructions and wear the appropriate personal protective equipment (PPE)
• prepare a risk assessment for the chemical and the site
• advise clients about safety risks, the treatment and precautions they can take to minimise risks
• ensure weather conditions are appropriate for application (see ‘Beaufort scale’).
• consider high-risk groups and the persistence of the pesticide and use the product with the lowest toxicity.

General precautions for all pesticides

There is a risk associated with the use of any pesticide, regardless of the chemical class.

However, the risks to the public and environment are minimised if you:

• read the label carefully and thoroughly
• follow the directions carefully
• take general precautions.

Currently, any pesticide available for use in Australia is registered with the Australian Pesticides and Veterinary Medicines Association. This ensures that products sold are safe and effective when used in accordance with the label directions.

High-risk groups

As a guide, where possible, the pesticide with the lowest toxicity should be used, particularly around high-risk groups such as:

• pregnant women
• infants
• children
• older people.

Persistence

If the pesticide has a high persistence, then consider where it is going to be applied, and the possible use of that area by adults, children and animals. The higher the half-life of the chemical, the more persistent it is.

Also consider that using pesticides indoors is different than using them outdoors, because the environmental conditions that normally accelerate pesticide breakdown such as sunlight, rainfall and soil microbes do not exist. This means that the pesticide may remain for longer indoors – that is, the persistence changes.

Concentrated pesticides

In general, pesticides are most toxic when in concentrated form, so particular care should be taken when handling and mixing undiluted chemicals. Appropriate PPE should be worn as outlined on the label and MSDS to prevent unnecessary exposure.

Advice to clients

Pest control operators (PCOs) have a wealth of knowledge about pests and the pesticides used to treat them. Clients generally have limited experience and may feel uneasy about the use of pesticides in and around their home. The PCO is their main source of information and advice.

PCOs should provide the following important information to their clients:

• the full name of the pesticide to be used and its toxicity rating
• potential health risks associated with its use, how it will be applied and to what areas
• the re-entry period if pesticides are used indoors
• pre- and post-treatment measures that the client can take to minimise exposure, which may include:
  • ensuring food, clothes, toys, toothbrushes, bedding, towels, vegetable gardens, barbeques, pet bowls, fish ponds, clotheslines and cooking utensils are covered or removed from the area to be treated
  • relocating pets during treatment and until the pesticide is dry
  • vacating the premises while the pesticide is mixed and applied, and until the pesticide is dry (generally 4–6 hours)
  • ensuring that all doors and windows are closed if the pesticide is to be applied outdoors
  • ventilating the house by opening all doors and windows upon return if a chemical smell can be detected
  • ensuring that benchtops and kitchen utensils are thoroughly cleaned before using them to prepare food if the pesticide has been applied indoors.

Organophosphates

Organophosphates (OPs) are used because they are very effective in killing a variety of pests. Pests do not appear to develop resistance to this class of pesticides as often as they do to other classes.

However, the use of OPs has diminished in favour of other pesticides. These alternative pesticides have shorter persistence and lower toxicity, making them more environmentally sound.

Common OP products used in pest control include (active ingredient is in brackets):

• Dursban (chlorpyrifos)
• Baytex 550 Spray (fenthion)
• Insectigas-D (dichlorvos).

Properties of organophosphates

OPs exist in a variety of forms, such as liquids, mists, baits, powders, pastes and dusts.

In general, OPs are extremely toxic, particularly in the concentrated form.

OPs are generally more persistent than other chemical classes currently used. Although some OPs start to breakdown within hours of application, others have a much higher persistence and will take longer to breakdown.

OPs can breakdown:

• chemically under the influence of environmental factors such as
  • sunlight
  • air
  • rainfall
  • soil moisture
• biologically through plants, animals and microorganisms like bacteria and fungi.

The more exposure OPs have to these elements, the faster they will breakdown.

Some labels and MSDSs contain information regarding the ‘half-life’ of the pesticide. This refers to the time that it takes for the concentration of the applied pesticide to reduce by half. If the half-life of a pesticide is one week, then after 7 days the concentration will be half that originally applied, after 2 weeks it will be a quarter, and so on.

Modes of action of organophosphates

OPs can be absorbed directly through the skin, the lining of the stomach or respiratory tract, after contact, ingestion or inhalation.

OPs affect the nervous system by attaching to the enzyme acetylcholinesterase. When functioning normally, nerves transmit messages through the production of a chemical called acetylcholine (ACh). After a message is sent, the enzyme acetylcholinesterase breaks down the ACh to end stimulation of the nerve and return it to its normal state.

OPs inhibit this enzyme, causing an accumulation of ACh and overstimulating the nerves. This causes the insect to lose control of their nervous system, resulting in weakness, paralysis and respiratory failure.

Because OPs affect acetylcholinesterase, they are known as ‘anti-cholinesterase compounds’.

Hazards of organophosphates

As the human nervous system also relies on acetylcholinesterase, they are also susceptible to OPs. If humans inhale, ingest or absorb enough OP pesticide, they are at risk of experiencing adverse health effects.

Acute symptoms of OP poisoning include:

• stomach cramps
• sweating
• muscle contraction
• twitching
• weakness.

Chronic poisoning from longer-term exposure could result in:

• general feelings of illness
• loss of appetite
• anaemia
• liver, kidney or nerve damage.

Carbamates

Carbamates are broad-spectrum pesticides that are effective against a variety of pests. Commonly used products containing carbamates include (active ingredient in brackets):

• Ficam D and Ficam W (bendiocarb)
• Blattanex Aerosol (propoxur and pyrethroid tetramethrin).

Properties of carbamates

Carbamates are considered slightly less toxic than OPs because they are rapidly processed by the body (metabolised) and excreted.

Carbamates also do not persist in the environment for as long as OPs, and usually breakdown within days to weeks. Certain environmental conditions will cause some carbamates to persist for longer.

Carbamates generally have low vapour pressure and low water solubility. This means that they are slow to evaporate and do not dissolve readily in water.

Modes of action of carbamates

Carbamates act mainly as contact and oral poisons, because they are absorbed readily through the skin, stomach lining or respiratory tract. They have a similar mode of action to OPs, which is to inhibit the functioning of acetylcholinesterase. This results in nervous system failure with symptoms similar to OP poisoning.

In humans, however, carbamates are considered to be reversible inhibitors, which means recovery from overexposure is typically faster than with OPs.

Carbamates do not remain in the body like other pesticides can, so the risk of chronic poisoning is minimised.

Hazards of carbamates

Carbamates are considered moderately toxic. Acute carbamate poisoning symptoms will usually begin within minutes of exposure and will last a few hours as the body works to metabolise the chemical.

Symptoms normally present as stomach cramps and sweating. However, if exposure continues, symptoms may be similar to OP poisoning:

• slurring of speech
• twitching and jerky movements
• difficulty breathing
• blurred vision
• weakness.

Chronic exposure may result in:

• appetite loss
• weakness
• weight loss
• general feeling of sickness.

Chronic poisoning is not as common with carbamates compared to OPs, because the body is able to metabolise and excrete carbamate-based pesticides.

Pyrethrins

Pyrethrum is a natural pest repellent found in flowers of certain Chrysanthemum species, such as daisies.

The active ingredient in pyrethrum is pyrethrin. There are different types of pyrethrins, the most common of which are pyrethrin I and pyrethrin II.

Pyrethrins are particularly effective against flying pests such as mosquitoes and flies, but can also be used against lice, fleas, silverfish, ants and cockroaches. The most commonly used pyrethrin pesticides are:

• Pestigas Pyrethrins Insecticide
• Py Fog
• Py Mist
• Py Spray
• Pymatic.

Properties of pyrethrins

Pyrethrins are thick, sticky, brown plant extracts. They may come in liquid or solid form, and inactivate readily in air. They are generally considered to be insoluble in water, but will dissolve in other chemicals, such as alcohol, oil or odourless kerosene.

Pyrethrins are usually mixed with synergists, which are added chemicals that do not have insecticidal properties on their own, but help the pyrethrins to work. For example, piperonyl butoxide is often added to pyrethrin pesticides to increase the toxicity of pyrethrin to pests.

Pyrethrins are not very stable in light and air, and are therefore not very persistent in the environment.

Modes of action of pyrethrins

Pyrethrins act on contact, quickly affecting the nervous system to ‘knockdown’ the pest. A few minutes after application the pest cannot move or fly away.

Pyrethrin I is highly lethal, whereas pyrethrin II has excellent ‘knockdown’ properties for a wide range of pests.

Pyrethrins are normally inhaled or ingested by the pest after contact and absorbed through the lining of the stomach or respiratory tract. They affect the nervous system by attaching to a protein found on the surface of nerves called the sodium channel.

This channel opens to stimulate the nerve and closes to end the signal when functioning normally. The pyrethrins bind to the sodium channel and prevent it from closing – thus overstimulating the nerve and causing pests to lose control of their nervous system.

This is evident in the tremors exhibited by affected pests as they lose coordinated movement.

Hazards of pyrethrins

Pyrethrins are much more toxic to pests than mammals. Mammals are able to breakdown pyrethrins into less toxic chemicals, which are then excreted.

However, if exposed to large quantities, humans may show symptoms of poisoning including:

• sneezing
• runny nose
• sore throat
• breathing difficulties.

Pyrethrins can cause nausea and vomiting if ingested in large quantities. Chronic pyrethrin exposure could result in liver damage.

Synthetic pyrethroids

Synthetic yrethroids are synthetic versions of the naturally occurring pyrethrins. Pyrethroids exhibit greater stability in the environment, and are therefore more persistent. They are also designed to target specific pest species.

More than 1000 synthetic pyrethroids have been produced with greater insecticidal activity than natural pyrethrins. Synthetic pyrethroids are effective on a wide range of pests. However, if used in excess, pests may become resistant to the pesticide.

The most commonly used synthetic pyrethroids include (active ingredient in brackets):

• Biflex Termiticide and Insecticide (bifenthrin)
• Coopex Insecticidal Dusting Powder (permethrin)
• Cislin Residual Insecticide (deltamethrin).

Properties of synthetic pyrethroids

Pyrethroids are insoluble in water, immobile in soil and have a high adsorption ability with particles such as wood and soil. This means that once applied, they will stick to the material and will not tend to transfer.

Pyrethroids are broken down by sunlight and microorganisms such as bacteria. Therefore, synthetic pyrethroids are generally not persistent in the environment, although they are more persistent than pyrethrins.

Pyrethroids were designed to be metabolised quickly in mammals. This means that they pose a reduced risk of poisoning to mammals, but remain toxic to insects.

Modes of action of synthetic pyrethroids

The mode of action of pyrethroids is identical to that of pyrethrins. They affect the sodium channels in nerve cells and cause overstimulation of the nervous system.

Hazards of synthetic pyrethroids

Synthetic pyrethroids are less toxic to mammals than pyrethrins, carbamates and organophosphates, although they do differ significantly in toxicity across the class.

Pyrethroids are not easily dissolved in water, but do adhere strongly to substances. There is a risk associated with spray drift affecting foodstuffs, such as vegetable gardens, and being consumed.

Pyrethroids are quite toxic to fish and other aquatic organisms, so care should be taken when using pyrethroids outdoors near waterways or fish ponds, or indoors near fish tanks.

Although pyrethroids do not persist in the body, poisoning can still occur when exposed to high levels. Exposure may result in symptoms such as:

• dizziness
• headache
• nausea
• vomiting
• eye irritation.

Although synthetic pyrethroids do not persist in the body, poisoning can still occur. An extremely high, single (acute) exposure to pyrethoids could lead to fatigue, muscular twitching and unconsciousness.

Chronic exposure to pyrethoids can include brain and nervous system disorders and immune system failures.