Toxic sludge-linked pollutant found in US air

Scientists tracking airborne particles over agricultural land in Oklahoma have identified medium-chain chlorinated paraffins, or MCCPs, in the atmosphere, marking what researchers describe as the first airborne detection of the chemical group in the Western Hemisphere. The finding has drawn attention because MCCPs are toxic industrial compounds under growing international scrutiny, and the researchers suspect the material may have reached the air from fertiliser made with treated sewage sludge, also known as biosolids.

The discovery emerged from a field campaign led by researchers at the University of Colorado Boulder in an agricultural region of Oklahoma. According to the peer-reviewed study published in ACS Environmental Au on 5 June 2025, the team reported real-time in-situ measurements of 18 gas-phase MCCPs in the southern Great Plains. Researchers had been studying how aerosol particles form and grow when instrument data showed unfamiliar isotopic patterns, which were later identified as chlorinated paraffins. University researchers said the measurements were taken continuously over about a month using a nitrate chemical ionisation mass spectrometer.

MCCPs are used in industrial applications including metalworking fluids, PVC and textiles. Scientists and regulators have been watching them more closely as restrictions on short-chain chlorinated paraffins, or SCCPs, have encouraged a shift towards substitute chemicals with similar functions. A 2022 review in Science of the Total Environment said concern has been rising over higher MCCP and LCCP production as replacements for SCCPs, while noting that airborne measurements remain sparse and technically difficult because of the compounds’ complexity and their tendency to associate with airborne particles.

That substitution effect is central to why the Oklahoma measurements matter. Researchers involved in the study said SCCPs, a related group, have long been recognised as persistent pollutants that can travel large distances and remain in the environment for extended periods. The Stockholm Convention process moved further against MCCPs as well: convention text published in 2025 states that production and use of medium-chain chlorinated paraffins shall be eliminated, subject to a range of specific exemptions for sectors such as construction, medical devices, aerospace, defence and some industrial uses. Earlier review committee materials also showed MCCPs had been recommended for listing in Annex A with exemptions for products such as PVC, adhesives, sealants and metalworking fluids.

The Oklahoma research does not claim to have proved a single source beyond doubt. But the authors pointed to biosolid fertiliser spread on nearby fields as a plausible pathway. MCCPs are often found in wastewater, researchers said, and can end up in sewage sludge after treatment. When that material is applied to fields, chemicals may be released back into the air. The lead author, Daniel Katz, said the team could not directly demonstrate that sewage sludge application caused the airborne readings, but described it as a reasonable explanation, adding that sludge-based fertilisers have been shown to release similar compounds.

That suspected route matters because biosolids have already become a sensitive environmental issue in the United States. Reuters reported in January 2025 that the Environmental Protection Agency warned treated sewage sludge used as fertiliser could pose health risks because biosolids can contain PFAS, the so-called forever chemicals. The agency’s draft assessment found possible risks above its thresholds for some people living near treated sites or consuming products from them, though it said the broader food supply was not implicated by that assessment. While PFAS and MCCPs are different chemical families, the policy debate over sludge application has widened scrutiny of what contaminants can persist in wastewater-derived farm inputs.

Evidence on chlorinated paraffins also suggests the health questions are serious enough to warrant caution. A 2023 scoping review on human health effects found chlorinated paraffins have been detected in blood, breast milk, placenta and other tissues, and said population-based and laboratory studies suggest links to liver and kidney toxicity, developmental toxicity, neurotoxicity, endocrine disruption, immune dysfunction and reproductive toxicity. The review also stressed that epidemiological evidence remains limited and that far more work is needed, especially for medium- and long-chain variants.