Acute toxicity refers to the immediate health effects (within 0-7 days) of exposure to a pesticide. It does not include long-term effects of pesticide exposure such as cancers or reproductive harms, which are often referred to as chronic toxicity.
Some pesticides are highly acutely toxic and can be lethal at very low doses. Regulatory authorities break acute toxicity into categories described by signal words such as Poison, Danger, Warning and Caution based on the LD50. LD50 is the dose in milligrams of substance per kilogram of body weight (or mg/kg) that kills 50% of animals tested through either oral or dermal exposure routes. For inhalation exposures, the LC50 is used, which is the concentration of substance in the air in milligrams per liter that kills 50% of the test animals.
Safety Data Sheets (SDSs) are produced by pesticide manufacturers, and provide toxicity information and emergency procedures for exposure to the chemical. If data are not available from the other sources, such as the product label, one can find toxicity information on the SDSs, most of which are available online.
PAN Acute Toxicity Categories
There are several organizations that evaluate and rank chemicals for their acute toxicity. Active ingredients of pesticides are ranked by the World Health Organization (WHO). Information on LD50 can be obtained from the U.S. EPA’s Toxics Release Inventory (TRI) ranking or manufacturers’ MSDS. Formulated pesticide products (which often include inert ingredients) are given a toxicity rating by the U.S. EPA which is shown as a warning label on the pesticide product.
Because toxicity designations from different sources sometimes conflict with each other, PAN created an acute toxicity designation that reflects the most toxic ranking assigned by any referenced authoritative source (excluding SDS). In addition, the different terms used by organizations to describe acute toxicity were translated into a consistent set of terminology. For example, if WHO determined a pesticide to be “Highly Hazardous,” we used the label “Highly Toxic.” The equivalences between the different ranking systems are shown in the table below.
| PAN category | Equivalence in other ranking systems |
| Extremely toxic | WHO: Extremely hazardous U.S. EPA: Category I, DANGER TRI: no parallel categoryGHS: Fatal if inhaled |
| Highly toxic | WHO: Highly hazardous U.S. EPA: Category I, DANGER TRI: Yes |
| Moderately toxic | WHO: Moderately hazardous U.S. EPA: Category II, WARNING TRI: no parallel category |
| Slightly toxic | WHO: Slightly hazardous U.S. EPA: Category III, CAUTION TRI: no parallel category |
| Not acutely toxic | WHO: Unlikely to be hazardous U.S. EPA: Category IV, CAUTION TRI: no parallel category |
Abbreviation key: WHO – World Health Organization, U.S. EPA – United States Environmental Protection Agency, TRI – Toxics Release Inventory (U.S. EPA), GHS – Globally Harmonized System
For a chemical to be classified as a PAN Bad Actor based on acute toxicity, it must be in either the Extremely Toxic or Highly Toxic PAN summary category.
World Health Organization Acute Hazard Rankings
WHO uses the Acute Toxicity Hazard Categories from the Globally Harmonized System (GHS) as the starting point for classification. WHO bases its ratings primarily on the lowest published oral or dermal LD50 in rat studies, the dose that kills 50% of the test animals in a standard assay.
While WHO ratings generally reflect acute toxicity, they also take into account other toxic effects such as reproductive and developmental toxicity. WHO does not evaluate fumigants, a class of gaseous pesticides that are generally extremely hazardous, nor does it evaluate pesticides believed obsolete or discontinued, even though some of these “obsolete” pesticides are currently registered for use in the United States.
EPA Acute Toxicity Rankings
Formulated pesticide products (which often include inert ingredients) are given an acute toxicity rating by EPA which is reflected in the warning label on the pesticide container. EPA gives a warning label of Category 1 to pesticide products with the highest acute toxicity levels and Category 4 to the least acutely toxic pesticide products. The toxicity categories are based on the LD50 and LC50 dose levels.
Active ingredients can also be ranked for toxicity on the basis of LD50 and LC50 values. Thus, warning labels for single-active-ingredient pesticide products containing technical grade active ingredients (over 90% pure) can serve as a reasonable proxy for the toxicity of the active ingredient. However, many of these warning labels are not internally consistent, and different pesticide products containing the same concentration of active ingredient are labeled with two or more different toxicity ratings. In this situation, EPA acute toxicity rating for the chemical was noted as “No Consensus Value.”
| U.S. EPA Categories and Warning Labels |
Acute Toxicity to Rats | |||||||
| Category | PAN Category | Warning Label | Oral LD50 (mg/kg) | Dermal LD50 (mg/kg) | Inhala- tion LC50 (mg/L) | Eye Effects | Skin Effects | |
| 1 | Highly Toxic | Danger- Poison* | ‹ 50 | ‹ 200 | ‹ 0.05 | —– | —– | |
| 1 | Highly Toxic | Danger | ‹ 50 | ‹ 200 | ‹ 0.05 | Corrosive (irreversible destruction of ocular tissue) or corneal involvement or irritation persisting for more than 21 days. | Corrosive (tissue destruction into the dermis and/or scarring) | |
| 2 | Moderately Toxic | Warning | 50-500 | 200-2,000 | 0.05-0.5 | Corneal involvement or irritation clearing in 8-21 days | Severe irritation at 72 hours (severe erythema or edema) | |
| 3 | Slightly Toxic | Caution | 500-5,000 | 2,000-5,000 | 0.5-2 | Corneal involvement or irritation clearing in 7 days or less | Moderate irritation at 72 hours (moderate erythema) | |
| 4 | Not Acutely Toxic | None | › 5,000 | › 5,000 | › 2 | Minimal effects clearing in less than 24 hours | Mild or slight irritation (no irritation or slight erythema) | |
*This signal word is used for acute systemic poisons.
All pesticide products registered for use in the U.S. are required to have an acute toxicity rating on the label. When there was a consistent acute toxicity rating for a given active ingredient, PAN assigned an acute toxicity rating. Where ratings were inconsistent or when acute toxicity was apparently due to an inert ingredient, no rating was assigned.
EPA Toxics Release Inventory List
In 1986, Congress passed the Emergency Planning and Community Right-To-Know Act (EPCRA) and the Pollution Prevention Act (PPA), with the intent of increasing transparency with respect to the use and disposal of chemicals in manufacturing, mining, and other activities. Section 313 of the EPCRA and section 6607 of the PPA required companies that release toxic materials to provide information to EPA on the identity and amounts of these toxic chemicals being released into the air, land and water. This information is made available to the public by the EPA as the Toxics Release Inventory (TRI), providing valuable information on the release and transport of toxic chemicals in the U.S.
A chemical is placed on the list if it is toxic to humans or damaging to the environment. The current TRI toxic chemical list contains approximately 650 individually listed chemicals and 30 chemical groups. Of this group, 350 individual chemicals and 25 chemical groups are pesticides or related compounds. Of these pesticide chemicals, only 222 have been formally ranked through the TRI program for their toxicity.
Toxicity categories evaluated for the TRI list of chemicals includes acute toxicity, carcinogenicity, reproductive and developmental toxicity, environmental toxicity and toxicity to organ systems including cardiovascular, liver, gastrointestinal, kidney, immune, hematological, and respiratory systems.
Globally Harmonized System
The international mandate that set the Globally Harmonized System (GHS) in motion was Agenda 21, adopted at the 1992 United Nations Conference on Environment and Development (UNCED). The goal was to create:
“A globally harmonized hazard classification and compatible labelling system, including material safety data sheets and easily understandable symbols.”
A GHS Sub-Committee, made up of representatives from various UN agencies, is maintains the GHS and provides additional guidance as needs arise. Below are the signal words and hazard ratings (for inhalation only) under the GHS system.
| Category 1 | Category 2 | Category 3 | Category 4 | Category 5 | |
| Signal Word | Danger | Danger | Danger | Warning | Warning |
| Inhalation | Fatal in inhaled | Fatal if inhaled | Toxic if inhaled | Harmful if inhaled | May be harmful if inhaled |
Resources:
WHO recommended classification of pesticides by hazard
U.S. EPA Pesticide Product Information System (PPIS)
Toxics Release Inventory and Community Right to Know, U.S. EPA.
Globally Harmonized System of Classification and Labelling of Chemicals (GHS)