Could the atmosphere above North America today contain the same industrial poisons previously believed to be restricted to factories and dump sites? Scientists at the University of Colorado Boulder have verified the first airborne detection of medium-chain chlorinated paraffins (MCCPs) in the Western Hemisphere an ominous milestone for environmental surveillance and chemical regulation.
MCCPs constitute one of a vast family of chlorinated paraffins, polychlorinated n-alkanes classified on the basis of carbon chain length: short-chain (C10–C13), medium-chain (C14–C17), and long-chain (C≥18). MCCPs find wide application as secondary plasticizers in paints, sealants, plastics, flame retardants, and metal cutting fluids. Their manufacture is colossal Europe alone registers 10,000–100,000 tonnes annually, and China alone manufactures more than a million tonnes of chlorinated paraffins of all kinds per annum, representing approximately 15% of worldwide production.
The recently released CU Boulder study, ACS Environmental Au, identified MCCPs in air samples taken from the ambient environment throughout the United States. The report is important because MCCPs exhibit numerous toxicological similarities to short-chain chlorinated paraffins (SCCPs), which were banned worldwide in 2017 as part of the Stockholm Convention due to their persistence, bioaccumulation, and toxicity. “We always have these unintended consequences of regulation, where you regulate something, and then there’s still a need for the products that those were in,” said Ellie Browne, a CU Boulder professor of chemistry and co-author. “So they get replaced by something.”
Toxicological data, summarized by the European Food Safety Authority, indicate that MCCPs act on the liver, kidneys, and thyroid in mammals. Repeated-dose studies have revealed enhanced kidney weights and histopathological alterations in rats and dogs. Rabbit and rat developmental studies revealed reduced pup viability and hemorrhage associated with vitamin K deficiency in neonates. Although acute toxicity is minimal, chronic dosing poses endocrine disruption, reproductive toxicity, and carcinogenic potential concerns. MCCPs have been detected in human blood, breast milk, and umbilical cord serum, suggesting maternal transfer to the fetus.
Their ability to undergo long-range atmospheric transport characteristic of persistent organic pollutants (POPs) is highlighted by the detection in open-air settings. Analytical developments have been instrumental in making such a discovery. The ability to identify MCCP congeners against the thousands of coincident mass fragments created by similar compounds is now made possible by high-resolution mass spectrometry and selective ion monitoring. These methods, developed in studies of plant uptake, have shown MCCP to accumulate in plants like maize, with leaf tissue being 60 times higher than in adjacent soils, indicative of atmospheric deposition as a pre-eminent route.
MCCPs equilibrate once in the environment among air, soil, water, and living organisms. They have been found in conifer needles, riverine sediments, and even in remote polar environments. In aquatic ecosystems, MCCPs inhibit development of fish and invertebrates, at times fatally. On land, they biomagnify and expose humans and wildlife to diet-based exposures. Stockholm Convention briefing reports that MCCPs “pollute the global environment, contaminate breast milk of women throughout the world, and pose dire health threats,” a sentiment echoed by IPEN Global Co-chair Pamela Miller, who cautioned that loopholes in proposed bans could delay contamination for decades.
Household indoor environments are another significant reservoir. Experiments in Norwegian homes identified MCCPs in 100% of settled dust samples, with median levels at 21 μg/g higher than in many other flame retardants. Inhalation and ingestion of dust are major exposure pathways; for MCCPs, dust ingestion contributes about 75% to indoor exposure, which is most concerning for toddlers because of hand-to-mouth behavior. Personal air sampling has demonstrated that true inhalation exposure may be five times greater than that estimated from stationary indoor air measurements, with contributions from the workplace, transportation, and other microenvironments.
The next 2025 Stockholm Convention conference on September 29 will discuss listing MCCPs for elimination worldwide. Industry lobbying, however, has already won a “long list of exemptions” for particular uses, prompting concerns over the efficacy of future regulation. The Convention’s own experts warned that ongoing production under exemption will create massive amounts of toxic waste, making remediation a more difficult process.
Locating MCCPs in air over the Western Hemisphere is merely the beginning. Scientists now must measure emission sources, learn about atmospheric transformation mechanisms, and map deposition patterns. These measurements will be essential for risk assessment as well as for developing monitoring schemes that can follow the effectiveness of a potential ban. As lead author Daniel Katz said, “We’re starting to learn more about this toxic, organic pollutant … which we need to understand better.”
