Washington State Department of Ecology

Department of Ecology News Release

FOR IMMEDIATE RELEASE - Sept. 28, 1999

99-190

Contact: Greg Sorlie, Hazardous Waste & Toxics Reduction Program, (360) 407-6702
Ron Langley, Public Information, (425) 649-7009

New study indicates farm soils have lowest levels of dioxins

OLYMPIA - Preliminary results of a new study by the Washington Department of Ecology (Ecology) show that agricultural soils in Washington have lower levels of dioxins than any other type of land in the state, including open land and forested areas.

Ecology randomly sampled lands that are currently being farmed to determine concentrations of dioxins typically found in agricultural soils. The median (or middle) concentration for dioxin found in Washington's agricultural soils, for example, was nearly 30 times lower than the same figure for urban soils.

Dioxin concentrations in Washington state soils by land use
Reported as Toxic Equivalents (TEQ)**, parts per trillion. Non-detect samples given a value of zero.

Land Use	Range	Average	Median	Geometric Mean*	Number of Samples
Urban	0.13 - 19	4.1	1.7	1.9	14
Forest	0.033 - 5.2	2.3	2.2	1.2	8
Open	0.040 - 4.6	1.0	0.27	0.24	8
Total (non-agricultural)	0.033 - 19	2.8	1.2	0.98	30
Agricultural	0.0078-1.2	0.14	0.054	0.062	54

*A geometric mean is similar to an average. In many environmental studies, including this one, a geometric mean is the best estimate of the central or most common value in a data set.

"While we don't have enough information to draw broad conclusions about dioxins in farm soils, we can say that land where Washington's crops are grown generally has less dioxin than other parts of the state," said Greg Sorlie, Ecology's hazardous waste manager. "The challenge for all of us now is to make sure it stays that way."

Sorlie said scientists are not certain why dioxin levels are so much lower in agricultural soils. The fact that farmlands usually are located far from urban sources of dioxins may contribute to lower levels. Tilling of agricultural land could be another factor because it may cause greater degradation and dilution of dioxins.

Ecology found no comparable data for dioxins in agricultural soils in the United States, but did find studies from Austria and Russia. Based on very limited data, it appears that agricultural soils in Washington are lower in dioxin than are soils in those countries, said Sorlie.

For all types of soil statewide, Ecology estimates that 1.4 parts per trillion represents the median (or middle) concentration of dioxin for all types of soils, taking into account the amount of land dedicated to each of the four major land uses in Washington (forested, open, agricultural and urban). This number compares favorably with the U.S. Environmental Protection Agency's estimated national average of eight parts per trillion dioxin in soils.

Data on dioxin levels in Washington's urban, open and forested areas were released in late 1998. Ecology's study of dioxins in agricultural soils was conducted this year, and a final report on those results is due in November.

Dioxins are unintended by-products formed when materials containing chlorine or chloride are burned, during chlorine bleaching of pulp and paper, or during the manufacture of pesticides and other chemicals. They are widespread in the environment and are commonly found in soils throughout the world at very low concentrations.

EPA has classified dioxins as "probable" human carcinogens, which means there is sufficient evidence from animal studies but insufficient evidence from human studies. Other non-cancer effects are also suspected.

*Editors Note about measurement of dioxins as "TEQs": There are 17 forms of dioxins considered to be toxic, but not all are equally toxic. The most toxic dioxin is called 2,3,7,8-TCDD, and other similar dioxins have been assigned toxicity values relative to it. These relative toxicity values are called toxicity equivalency factors (TEFs). 2,3,7,8-TCDD is assigned a TEF of 1, and the others are assigned values less than 1. The concentration and TEF for each form of dioxin are then factored into a single value that represents the overall toxicity in the environment, expressed as TEQ, or "toxic equivalent."