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Polycyclic aromatic hydrocarbons (PAHs) are a family of more than 100 chemicals that form when coal, oil, gas, wood, garbage and other organic material burn incompletely [1], and in soil they cling to particles. Microbes can break them down, at a speed set mostly by molecule size. In one laboratory soil study an added bacterium cut naphthalene by 63% and phenanthrene by 81% in 25 days, and the larger benzo[a]pyrene by only 38% [2]. The heaviest PAHs usually need more than one kind of microbe and more time [3].
Below: what PAHs are, where they turn up, how they behave in soil, what peer-reviewed research shows about biological breakdown, and where that research stops. The full contaminant profile is at PAHs and SVOCs. For comparison with other contaminant classes, see our fit chart of what microbes can break down; for petroleum, microbes that eat oil.
What PAHs are
A PAH is a molecule of two or more fused benzene rings [4]. Each added ring makes the molecule heavier, less soluble and more tightly held by soil. Labs and regulators usually split the family in two.
| Group | Rings | Examples | How microbes handle them |
|---|---|---|---|
| Low molecular weight | 2 to 3 | Naphthalene, phenanthrene | Broken down more easily [2] |
| High molecular weight | 4 to 6 | Pyrene, benzo[a]pyrene | Slower; often need more than one organism [3] |
PAHs fall inside the wider class of semivolatile organic compounds (SVOCs), and a soil report for “SVOCs” will usually list the individual PAHs by name.
Where PAHs come from
ATSDR, the federal toxic-substances agency, traces PAHs to incomplete burning of coal, oil, gas, garbage and other organic material, plus volcanoes, forest fires, coal burning and car exhaust [1]. In soil that covers a lot of ground:
- Former manufactured-gas plants, creosote yards and wood-treating sites are classic sources, and the bacterium and fungus in one well-known PAH study came from creosote-contaminated and gas-plant soils [3].
- Rail corridors, roadsides and industrial yards collect PAHs where fuel, soot and coal residue pile up over decades.
- Burned land can carry them too, and after the 2025 Palisades Fire, Los Angeles County reported one isolated area with PAHs above screening levels and said the source, fire-related or not, was unknown [5]. Our post on wildfire ash and metals covers that testing.
City soil carries background PAHs from ordinary combustion, so a detection alone doesn’t identify a source.
How PAHs behave in soil
ATSDR notes that PAHs in soil mostly bind tightly to particles, though some move down into groundwater [1]. A PAH locked onto clay or organic matter is hard for microbes to reach. Soil scientists call this low bioavailability, and it is the main reason old contamination is harder to treat than a fresh release.
Breakdown speed also depends on pH, temperature, oxygen, the microbes present, nutrients and the PAH’s own structure [4]. ATSDR gives weeks to months for microorganisms to break down PAHs in soil or water [1]. Heavy PAHs in aged soil sit at the long end of that range, or past it.
How microbes break PAHs down
Most PAH breakdown uses oxygen, with bacterial enzymes called oxygenases that attach oxygen to the aromatic rings while other enzymes open the rings, and the fragments go into ordinary metabolism. Complete breakdown ends in carbon dioxide and water [4]. Breakdown without oxygen has also been observed [4].
A literature review lists Pseudomonas, Mycobacterium, Rhodococcus and Paenibacillus among the commonly studied PAH-degrading bacteria. On the fungal side it lists white-rot fungi such as Phanerochaete chrysosporium and Pleurotus ostreatus, which attack PAHs from outside the cell with enzymes like laccase and manganese peroxidase [4]. The same review notes that compost added to contaminated soil can speed biodegradation [4].
The heavy PAHs need partners
Benzo[a]pyrene, with five rings, is one of the hardest to break. In a 2000 study, researchers isolated a soil bacterium and a fungus (Penicillium janthinellum) from separate creosote and gas-plant soils [3]. On its own each made little progress; together, in contaminated soil, they converted 53% of added radiolabeled benzo[a]pyrene to carbon dioxide in 100 days, and the soil extracts became less mutagenic than extracts from soil treated with the native microbes or with either organism alone [3].
The mutagenicity result deserves attention, because a lower concentration only shows the chemical went somewhere, while a less mutagenic extract points to lower toxicity, and few studies report it.
Adding a degrader to soil
In a 2022 lab study, researchers added Comamonas testosteroni to PAH-contaminated soil. By day 25, naphthalene was down 63.38%, phenanthrene 81.18% and benzo[a]pyrene 37.98%. The added bacterium also raised the numbers of native PAH-degrading bacteria, so it worked with the soil community [2]. The authors call it laboratory-level work.
The evidence at a glance
| Study | Organism | Setting | Reported result |
|---|---|---|---|
| Lu et al. 2022 [2] | Comamonas testosteroni | PAH-contaminated soil, lab | Naphthalene 63%, phenanthrene 81%, benzo[a]pyrene 38% at day 25 |
| Boonchan et al. 2000 [3] | A soil bacterium with Penicillium janthinellum | Contaminated soil, lab | 53% of added benzo[a]pyrene mineralized in 100 days; lower mutagenicity |
| Haritash and Kaushik 2009 [4] | Many | Literature review | Microbial degradation is the major PAH removal process; rate depends on conditions |
What to expect
Biology can treat PAHs, but slowly, over months and sometimes longer, with progress confirmed by lab testing.
- Heavy PAHs in aged, dry or tightly bound soil may decline slowly or only partway.
- Oxygen, moisture and nutrients have to be in range, and compacted or waterlogged soil stalls the process.
- A concentrated source such as a coal-tar pocket may need excavation or another method first, with biology finishing the lower-level residue.
- Success is a lab result against the regulator’s target, measured by someone other than us.
Microbes in Elm Dirt’s products with PAH research
Peer-reviewed research shows Comamonas testosteroni can degrade PAHs in soil [2]. It is one of the 291 microbial species identified in Elm Dirt’s Plant Juice by independent lab analysis (Biome Makers, 29 May 2024) [6]. Several Pseudomonas species, a genus on the commonly studied PAH-degrader list [4], are also among the 291; see Pseudomonas in soil remediation. The organism has its own profile under Comamonas testosteroni.
The studies used specific strains, while DNA sequencing identifies species, so the research shows what organisms of this species can do and results on a site are confirmed by lab testing.
Your site
PAH mix, soil type and site history differ everywhere. A treatability test on your actual soil comes before any field decision, and follow-up sampling across seasons shows whether the gains hold. If you have PAH or SVOC results in hand, we can go through them with you.
Sources
All links checked 2026-10-01.
- Agency for Toxic Substances and Disease Registry. ToxFAQs for Polycyclic Aromatic Hydrocarbons (PAHs). wwwn.cdc.gov/TSP/ToxFAQs
- Lu Q, Sun X, Jiang Z, Cui Y, Li X, Cui J. Effects of Comamonas testosteroni on dissipation of polycyclic aromatic hydrocarbons and the response of endogenous bacteria for soil bioremediation. Environmental Science and Pollution Research 29:82351 to 82364 (2022). doi:10.1007/s11356-022-21497-z
- Boonchan S, Britz ML, Stanley GA. Degradation and mineralization of high-molecular-weight polycyclic aromatic hydrocarbons by defined fungal-bacterial cocultures. Applied and Environmental Microbiology 66(3):1007 to 1019 (2000). doi:10.1128/AEM.66.3.1007-1019.2000
- Haritash AK, Kaushik CP. Biodegradation aspects of polycyclic aromatic hydrocarbons (PAHs): a review. Journal of Hazardous Materials 169(1 to 3):1 to 15 (2009). doi:10.1016/j.jhazmat.2009.03.137
- Los Angeles County Department of Public Health. Public Health Releases Final Findings of Soil Testing in Fire-Impacted Areas. Press release, 12 September 2025. publichealth.lacounty.gov
- Biome Makers (BeCrop). Independent lab analysis of Elm Dirt Plant Juice (Biome Makers lab profile), 29 May 2024. Report disclaimer: results are an interpretation of the potential function of the sample microbiome and are for research purposes.