Dioxins and Furans in Soil, Explained

Dioxins and furans are chlorinated compounds that form as unwanted by-products when chlorine-containing material burns, and in some manufacturing processes [1]. They bind tightly to soil particles and break down slowly [2]. Labs usually report them as a TEQ, one number that weights each compound by its toxicity relative to the most toxic, 2,3,7,8-TCDD [3]. TEQ and total mass measure different things, so two dioxin numbers for the same soil can look far apart and both be right.

Below: the vocabulary, how to read a dioxin result, and what peer-reviewed research shows about biological breakdown. The full contaminant profile is at dioxins and furans.

What dioxins and furans are

The World Health Organization describes dioxins as a group of chemically related, persistent environmental pollutants. About 419 related compounds exist, roughly 30 with significant toxicity, TCDD the most toxic [1]. There are two branches: polychlorinated dibenzo-p-dioxins (PCDDs, “dioxins”) and polychlorinated dibenzofurans (PCDFs, “furans”). EPA’s toxicity-equivalence document lists seven PCDDs and ten PCDFs with dioxin-like toxicity [3].

WHO’s list of sources includes manufacturing by-products such as smelting and chlorine bleaching of paper pulp, along with volcanic eruptions and forest fires [1]. In published soil work, the sources that come up are combustion involving chlorine and sites tied to chlorinated chemicals, such as a closed pentachlorophenol plant [10].

How dioxins behave in soil

ATSDR reports that chlorinated dibenzo-p-dioxins have large soil adsorption coefficients, low mobility, an affinity for particles and long persistence [2]. In practice they stay near where they landed, move mostly with soil particles as dust or erosion rather than dissolved in water, and are hard for microbes to reach.

Reading a dioxin result: TEQ versus total

A lab can report the mass of each dioxin and furan, called congeners, in a sample. Seventeen separate numbers are hard to compare, so EPA and WHO use toxic equivalency factors (TEFs). Each congener’s potency is compared with 2,3,7,8-TCDD, which gets a factor of 1. Multiply each concentration by its TEF and add them up, and the sum is the toxic equivalent, or TEQ [3][4].

EPA’s recommended factors, which follow the 2005 WHO values, include [3][4]:

WHO re-evaluated the factors in 2022, so exact values vary between documents and years [5].

A worked example

The arithmetic below uses the factors above and is for illustration only, with no site data involved.

• Sample A has 2.0 pg/g of 2,3,7,8-TCDD and nothing else, so its total and its TEQ are both 2.0 pg/g.
• Sample B has 6,000 pg/g of OCDD and nothing else, which gives a total of 6,000 pg/g and a TEQ of 6,000 x 0.0003 = 1.8 pg/g.

Sample B holds 3,000 times more dioxin by mass and has the lower TEQ. One report quoting total mass and another quoting TEQ can both be accurate and still sound like they disagree.

Units confuse things further, since one picogram per gram (pg/g) equals one nanogram per kilogram (ng/kg) and one part per trillion (ppt). A value in parts per billion is 1,000 times larger.

Other reasons two numbers differ

• Reports measure different congeners, with some covering 17 and others adding dioxin-like PCBs.
• Labs and agencies handle non-detects, the congeners below the detection limit, differently when totaling a TEQ.
• Depth and location matter, because a surface sample and a 6-inch sample can differ, as can a roadside sample and a field sample.
• Long-running combustion leaves low background levels of dioxin in many soils, so readings are usually compared with local background, which itself varies.
• EPA publishes regional screening levels (RSLs) for dioxin TEQ in soil and states that RSLs are not cleanup standards and should not be used as cleanup levels [6], so exceeding one triggers a closer look.

For these differences in a published case, see our guide to East Palestine soil data.

What biology has been shown to do

Each result below comes from a laboratory or treatability study. None is a regulatory finding that biology can clear a dioxin site.

Aerobic bacteria have the clearest record on the lighter congeners, and a review notes that aerobic microbial degradation of dioxins and furans is reported mainly for low-chlorinated congeners [7]. In a 1999 soil microcosm study, a single added bacterial strain brought dibenzofuran, dibenzo-p-dioxin and 2-chlorodibenzo-p-dioxin (10 ppm each) down to the parts-per-billion range, with the chlorinated dioxin’s half-life running from 5.8 hours in soil with 0% organic matter to 26.3 hours at 5.5% [8]. These were simple, lightly chlorinated compounds, not the toxic 2,3,7,8-substituted congeners, and the lightly chlorinated end is where biology’s record is strongest.

For the heavier congeners the evidence points to anaerobic bacteria. In 2003, researchers showed a Dehalococcoides strain reductively dechlorinating selected dioxin congeners, including the environmentally significant 1,2,3,7,8-PeCDD [9]. A later review suggests reductive dechlorination may be one of very few mechanisms that work on polychlorinated dioxins, lowering their toxicity and leaving lower-chlorinated products that can break down further [7].

White-rot fungi have also been tested, and a 2020 study treated real field soil from a closed pentachlorophenol plant with a white-rot fungus and reported 96% overall PCDD/F removal in 72 days [10]. The design and limits are in a separate Study Breakdown, and the fungal group in white-rot fungi in soil remediation.

What biology cannot promise

• These are weeks-to-months results under controlled conditions, and in the field the work takes months to a year or more, verified by lab testing.
• Biology won’t reach every congener equally, and the heavily chlorinated and 2,3,7,8-substituted forms are the hardest.
• No outcome is set in advance, because organic matter, congener mix, moisture and temperature all change the result.
• Where a regulator requires removal, biology is no substitute for excavation.

Microbes in Elm Dirt’s products and dioxins

Peer-reviewed research shows two bacteria identified in Elm Dirt’s products by independent lab analysis (Biome Makers) can act on the dioxin and furan ring. The biphenyl dioxygenase of Comamonas testosteroni oxygenates both dibenzofuran and dibenzo-p-dioxin [12]. A Pseudomonas putida strain grown on biphenyl cometabolically degrades dibenzofuran, the unchlorinated parent compound [13]. Profiles are on our dioxins and furans page.

In the field, biology is paired with binding. Activated carbons and biochars added to dioxin-contaminated soil cut the measured availability of total dioxins and furans by 40 to 92 percent, confirmed by earthworm uptake [11]. The dioxin remains, but less of it can reach plants, animals and people. Elm Dirt’s approach combines soil biology, biochar and carbon amendments, with independent lab monitoring over time.

Open questions

How a particular soil’s congener mix, organic matter and history would affect a biological approach can only be learned by treatability testing on that soil. It is also unclear how well bench results carry over to field soil near background concentrations. If you have dioxin results and want help reading them, send them our way.

Sources

All links checked 2026-10-01.

1. World Health Organization. Dioxins (fact sheet). who.int
2. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Chlorinated Dibenzo-p-Dioxins (CDDs). atsdr.cdc.gov/toxprofiles/tp104.pdf
3. U.S. EPA Risk Assessment Forum. Recommended Toxicity Equivalence Factors (TEFs) for Human Health Risk Assessments of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Dioxin-Like Compounds. EPA/100/R-10/005 (December 2010). semspub.epa.gov
4. Van den Berg M, Birnbaum LS, Denison M, et al. The 2005 World Health Organization reevaluation of human and mammalian toxic equivalency factors for dioxins and dioxin-like compounds. Toxicological Sciences 93(2):223 to 241 (2006). doi:10.1093/toxsci/kfl055
5. DeVito M, Bokkers B, van Duursen MBM, et al. The 2022 World Health Organization reevaluation of human and mammalian toxic equivalency factors for polychlorinated dioxins, dibenzofurans and biphenyls. Regulatory Toxicology and Pharmacology 146:105525 (2024). doi:10.1016/j.yrtph.2023.105525
6. U.S. EPA. Regional Screening Levels (RSLs). epa.gov/risk/regional-screening-levels-rsls
7. Bunge M, Lechner U. Anaerobic reductive dehalogenation of polychlorinated dioxins. Applied Microbiology and Biotechnology 84:429 to 444 (2009). doi:10.1007/s00253-009-2084-7
8. Halden RU, Halden BG, Dwyer DF. Removal of dibenzofuran, dibenzo-p-dioxin, and 2-chlorodibenzo-p-dioxin from soils inoculated with Sphingomonas sp. strain RW1. Applied and Environmental Microbiology 65(5):2246 to 2249 (1999). doi:10.1128/AEM.65.5.2246-2249.1999
9. Bunge M, Adrian L, Kraus A, et al. Reductive dehalogenation of chlorinated dioxins by an anaerobic bacterium. Nature 421:357 to 360 (2003). doi:10.1038/nature01237
10. Kaewlaoyoong A, Cheng CY, Lin C, Chen JR, Huang WY, Sriprom P. White rot fungus Pleurotus pulmonarius enhanced bioremediation of highly PCDD/F-contaminated field soil via solid state fermentation. Science of the Total Environment 738:139670 (2020). doi:10.1016/j.scitotenv.2020.139670
11. Chai Y, Currie RJ, Davis JW, et al. Effectiveness of activated carbon and biochar in reducing the availability of polychlorinated dibenzo-p-dioxins/dibenzofurans in soils. Environmental Science & Technology 46(2):1035 to 1043 (2012). doi:10.1021/es2029697
12. L’Abbée JB, Barriault D, Sylvestre M. Metabolism of dibenzofuran and dibenzo-p-dioxin by the biphenyl dioxygenase of Burkholderia xenovorans LB400 and Comamonas testosteroni B-356. Applied Microbiology and Biotechnology 67(4):506 to 514 (2005). doi:10.1007/s00253-004-1791-3
13. Li Q, Wang X, Yin G, et al. New metabolites in dibenzofuran cometabolic degradation by a biphenyl-cultivated Pseudomonas putida strain B6-2. Environmental Science & Technology 43(22):8635 to 8642 (2009). doi:10.1021/es901991d