Triazine Herbicides

(Atrazine, Cyanazine, Propazine, Simazine, others)

Triazine Herbicides

The triazine herbicides are a set of related compounds that can be used to some degree interchangeably, and have similar adverse effects. Chemically, they all have a triazine ring. The most widely used are atrazine, simazine, cyanazine, and propazine, though there are others as well. Atrazine is the second most widely used agricultural pesticide in the U.S., and cyanazine is in the top five. They kill plants by inhibiting photosynthesis, and are generally used as pre-emergence broad-leaf weed killers.


Most atrazine is used on field corn and sorghum. However, it has a wide range of other uses, including turf and lawns, pineapples, sugarcane, wheat, forestry, and soybeans. It was used in the U.S. until 1993 for control of vegetation in fallow and noncrop land.

Cyanazine is used on corn, cotton and sorghum, with an estimated 95 percent used on corn. Simazine's largest single use is on corn, but it is also used on a variety of deep-rooted crops such as artichokes, asparagus, berry crops, broad beans, citrus, and fruit orchards, as well as in farm ponds, fish hatcheries, and in lawn care. In 1994, all swimming pool uses of simazine were ended in the U.S. due to EPA's concern about human exposure.

Because of an ongoing EPA review of the triazine herbicides, cyanazine will be phased out of production for domestic use by the end of 1999, with use of remaining stocks allowed until 2002. Other restrictions on triazines may follow as the review continues.

Health Effects

Triazine herbicides are classed as possible human carcinogens, based on an increase in mammary gland tumors in female laboratory rats. EPA has determined that they may pose significant risks of cancer to people who are highly exposed through residues in food and water and to people who apply the herbicides. This is EPA's principle current reason for regulating these chemicals. Preliminary results show a correlation between high atrazine exposure and heart and limb birth defects in one small geographic area. Triazines can cause harmful acute effects from a single very high dose. In addition, atrazine can cause cardiovascular damage and retinal and muscular degeneration from chronic high exposure. Simazine can cause tremors and damage to liver, kidneys, and thyroid from a long exposure above safe levels. Of more concern for low-level exposure, the triazine herbicides are thought to be endocrine disruptors.

In the Environment

Unlike the other PBTs in this report, the triazine herbicides are persistent but not very bioaccumulative. Simazine has a low potential to bioaccumulate in fish, but atrazine does not. These chemicals are found at high levels not because of their bioaccumulation, but because of their persistence and ongoing large-scale use. These herbicides are very mobile in soil, which means that a heavy rain can wash them into nearby water bodies. They are commonly found in drinking water in the spring and summer in U.S. Midwest farming areas. Atrazine was the second most frequently detected pesticide in a survey of drinking water wells.

Atrazine has a half-life in soil from 1 week to 1 year, depending on soil conditions. It may be significantly degraded by microbes in soil. Simazine can last for up to three years in soil under aquatic conditions. These compounds are thought to be relatively persistent in water.

Weeds that suffer ongoing heavy use of a triazine herbicide can become resistant. This appears to happen after about 10 years of good control. Triazines are fatal to some aquatic plants at very low concentrations, and for this reason they may be an ecological concern throughout the corn belt of the U.S.


Herbicides are released to the environment through normal use, so it should be expected that almost all of the amount produced will eventually be released. Sources include the manufacturing facilities for the herbicides, their distributors, farm use, and commercial and home use for lawn care.

In the U.S., the most popular triazine pesticides are made by a few large manufacturers. Ciba-Geigy of Greensboro, NC was the principal manufacturer of atrazine and simazine. Since Ciba-Geigy's merger with Sandoz, their corporate identity for atrazine and simazine manufacture has become Novartis Crop Protection. DuPont was the principal manufacturer of cyanazine, although Griffin Corp. is now also involved with the chemical. Griffin is the principal registrant for propazine.

Recent (1994) EPA estimates of triazine active ingredient usage in the U.S. are 90 to 120 million pounds per year. About 64 to 80 million pounds of this is atrazine, 21 to 34 million is cyanazine, and 5 to 7 million is simazine. In addition, about 200,000 to 400,000 pounds of propazine is used per year on sorghum through emergency exemptions. Because of the cyanazine phase-out, use of that chemical is declining, with estimated annual use probably falling to 21 to 25 million pounds. Estimates of annual triazine use based on 1991-1993 data by USGS are consistent with EPA's, with 64 million pounds for atrazine, 29.5 million for cyanazine, and 4.6 million for simazine for a total of 98 million pounds.

Atrazine, cyanazine, and simazine are listed in EPA's Toxic Release Inventory (TRI) from 1995 on, but these releases from large single facilities are small in comparison to the total agricultural and lawn use of these chemicals. For 1995, 660,000 pounds of atrazine were released, mostly to land, while 230,000 pounds were transferred elsewhere. Cyanazine had only 1,500 pounds of TRI release and 10,000 pounds of transfers in 1995, and simazine had 5,000 pounds of releases and 26,000 pounds of transfers.


* Environmental Protection Agency. National Primary Drinking Water Regulations: Technical Factsheet on Atrazine. Office of Ground Water and Drinking Water. January 1998.

* Environmental Protection Agency. National Primary Drinking Water Regulations: Technical Factsheet on Simazine. Office of Ground Water and Drinking Water. January 1998.

* Environmental Protection Agency. Office of Pesticide Programs Annual Report for 1995. Office of Prevention, Pesticides, and Toxic Substances. EPA-730-R-95-002. December 1995.

* University of Minnesota. Atrazine Pathway Map (web page). Yuemo Zeng, Colin L. Sweeney, and Stephen Stephens. March 4, 1998.

* U.S. Geological Survey. National Water Quality Assessment Pesticide National Synthesis Project: Annual Use Maps for atrazine, cyanazine, and simazine. March 20, 1998.

* Agriculture Western Australia. Implications of Herbicide Resistance on Weed Management in Canola. Julie Cooper.

* Environmental Protection Agency. Federal Register -- Atrazine, Simazine and Cyanazine; Notice of Initiation of Special Review. Federal Register, November 23, 1994.

* Federal Trade Commission. Federal Register -- Ciba-Geigy Limited, et al.; Analysis to Aid Public Comment. Federal Register, January 3, 1997. Volume 62, Number 2, Page 409-414.

* Environmental Protection Agency. Pesticides and Ground Water SMP Rule Technical Support: Summary of Evidence of Adverse Human Health Effects

for 5 SMP Candidates. Office of Pesticide Programs. June 1996.

* Environmental Protection Agency. International Pesticide Notice: Cyanazine Pesticide Voluntarily Canceled and Uses Phased Out (web page).

* Environmental Protection Agency. Questions & Answers: the Triazine Pesticides: Atrazine, Cyanazine, Propazine, and Simazine (web page). Office of Pesticide Programs. August 1997.

* Environmental Protection Agency. Federal Register -- Cyanazine; Notice of Final Determination to Terminate Special Review. Federal Register: July 25, 1996. Volume 61, Number 144, Page 39023-39029.

* Environmental Protection Agency. Toxic Release Inventory Database (1996 "frozen" version).

* Colborn, T., F.S. vom Saal and A.M. Soto. 1993. Developmental effects of endocrine-disrupting chemicals in wildlife and humans. Environmental Health Perspectives 101:378-384.