The half-lives of pesticides can be grouped into three groups to estimate persistence. These are low (half-life less than 16 days), moderate (16 to 59 days) and high (more than 60 days). Pesticides with shorter half-lives tend to accumulate less because they are much less likely to persist in the environment. Pesticides, such as herbicides, fungicides and insecticides, vary in the time they decompose in the environment depending on the specific pesticide, the speed applied and the environmental conditions.
We measure how long pesticides stay in the environment using a measure called half-life or how long it takes for the original material to be reduced by 50%. In most situations that we would encounter in an agricultural environment, the average life of a pesticide can range from a few hours to 4-5 years. Most pesticides are broken down by soil microbes, so environmental conditions that reduce microbial activity (cold, dry conditions) will prolong the pesticide's stay in the soil. Overall, the trend is for newer pesticides to last much less than those used decades ago (e.g.
DDT). In some parts of the world, copper-based fungicides are still used, which will last forever in the soil, for all practical purposes. In fact, most herbicides sold in homes are required by law to decompose in soil within 14 days, if not sooner. This non-selective, post-emergent herbicide generally breaks down within days or weeks, depending on the specific product you have.
In addition to the type of surface, the duration of application of an insecticide is also affected by temperature, light, air movement and humidity. In general, insecticides applied in warm environments, such as near ovens or in some kitchens, break down more quickly. Bright sunlight and high-intensity ultraviolet light also accelerate the breakdown of many insecticides. The residual life of an indoor insecticide is further affected by moisture in the form of evaporation, condensation or high humidity.
Cleaning procedures will remove insecticides from surfaces. In a place with high heat and high humidity, the application of an insecticide may not last long. A typical pest control application will last around 90 days. If outdoor applications produce constant or heavy rainfall, their effectiveness may be affected and will last for about 60 days.
Pesticides used to treat flying insects, such as mosquitoes or flies, will last for about 30 days. For a general preventive application of pesticides, it is best to do what professionals do and apply them every 90 days for continuous monitoring. Because children grow quickly, they “introduce more of the pesticides found in food, water and air into their bodies,” she says. These pesticides are those that do not offer significant residue and must come into direct contact with the pest to be effective.
When wondering how long a pest control application lasts, you should consider whether the pesticide you are using is a contact or residual pesticide or not. When pests crawl through the residual pesticide, they will pick it up on their legs or after examining the surface with their antennae or parts of the mouth. Some pesticides enter through vents and windows during and after fumigation; people and dogs also track waste. The truth is that it depends on several factors, such as the pest being treated, the product being used, the place where they are applied and even the active ingredient of the pesticide being used.
Most people “aren't aware of the dangers that the thoughtless use of pesticides poses to their children,” says Philip Landrigan, dean of Global Health and professor of pediatrics at Mount Sinai School of Medicine in New York. When a residual pesticide is applied to a surface, it leaves behind the active ingredient of the pesticide that will continue to control insects. Re-entry intervals are set to protect people (and animals, for example, in a kennel) from pesticide poisoning if they enter the treated area too soon after application without appropriate protective equipment. Workers who are trained and certified in pesticide management may be allowed to enter the treated area for short-term work before the end of the re-entry interval.