Environmentally Persistent Free Radicals and Their Impact on Reactive Oxygen Species

Environmentally persistent free radicals (EPFRs) are stable radical species that form on particulate matter surfaces during combustion or thermal processes. Their prolonged lifetimes enable long-range transport and sustained reactivity. By redox cycling with atmospheric oxygen or biological reductants, EPFRs generate reactive oxygen species (ROS) such as superoxide and hydroxyl radicals. This continuous ROS production can overwhelm antioxidant defences in cells and tissues, leading to oxidative damage, inflammation and impaired function. EPFRs arise from diverse sources—wildfire charcoals, industrial emissions, vehicle exhausts and contaminated soils—and are implicated in respiratory and cardiovascular diseases, ecological toxicity and the ageing of materials. Recent advances have elucidated how particle composition, surface chemistry and environmental conditions govern EPFR formation, stability and decay, informing risk assessment and remediation strategies on a global scale.

Research from Nature Portfolio

Analyses of wildfire-derived charcoals across boreal, temperate and tropical climates reveal extremely high concentrations of EPFRs that persist for at least five years. Electron spin resonance measurements show that charcoals from woody fuels and higher degrees of carbonisation yield the greatest radical loads, suggesting that wildfires are a globally significant EPFR source capable of sustained ROS generation. In parallel, studies of hydrothermal carbonisation of sewage sludge demonstrate that process parameters—temperature in particular—critically influence the formation of oxygen-centred and carbon-centred EPFRs. Under moderate temperatures (120–150 °C), oxygen-centred radicals dominate, whereas at elevated temperatures (260–280 °C) carbon-centred radicals emerge, both exhibiting lifetimes of months to over a year. These insights link waste-treatment practices to EPFR emissions and inform process optimisation to minimise radical generation.

Environmentally Persistent Free Radicals and Their Impact on Reactive Oxygen Species publication trend

The graph below shows the total number of articles in environmentally persistent free radicals and their impact on reactive oxygen species across all publications each year (not limited to Nature Index journals).

Technical terms

Environmentally persistent free radicals (EPFRs): radicals stabilised on particle surfaces with prolonged environmental lifetimes that catalyse redox reactions.

Reactive oxygen species (ROS): highly reactive molecules derived from oxygen that can damage biological macromolecules and trigger signalling pathways.

Electron paramagnetic resonance (EPR): spectroscopic technique for detecting unpaired electrons and characterising radical species on solid surfaces or in solution.

Aryl hydrocarbon receptor (AhR): ligand-activated transcription factor that mediates cellular responses to environmental contaminants and modulates oxidative stress and inflammation.

References

1. Environmentally persistent free radicals amplify ultrafine particle mediated cellular oxidative stress and cytotoxicity. Particle and Fibre Toxicology (2009).
2. Particulate matter containing environmentally persistent free radicals induces AhR-dependent cytokine and reactive oxygen species production in human bronchial epithelial cells. PLOS ONE (2018).
3. Environmentally persistent free radicals are ubiquitous in wildfire charcoals and remain stable for years. Communications Earth & Environment (2021).
4. Assessing the effect on the generation of environmentally persistent free radicals in hydrothermal carbonization of sewage sludge. Scientific Reports (2019).
5. Size-resolved exposure risk of persistent free radicals (PFRs) in atmospheric aerosols and their potential sources. Atmospheric Chemistry and Physics (2020).