Addressing Emerging Risks: Scientific and Regulatory Challenges Associated with Environmentally Persistent Free Radicals

Abstract

Airborne fine and ultrafine particulate matter (PM) are often generated through widely-used thermal processes such as the combustion of fuels or the thermal decomposition of waste. Residents near Superfund sites are exposed to PM through the inhalation of windblown dust, ingestion of soil and sediments, and inhalation of emissions from the on-site thermal treatment of contaminated soils. Epidemiological evidence supports a link between exposure to airborne PM and an increased risk of cardiovascular and pulmonary diseases. It is well-known that during combustion processes, incomplete combustion can lead to the production of organic pollutants that can adsorb to the surface of PM. Recent studies have demonstrated that their interaction with metal centers can lead to the generation of a surface stabilized metal-radical complex capable of redox cycling to produce ROS. Moreover, these free radicals can persist in the environment, hence their designation as Environmentally Persistent Free Radicals (EPFR). EPFR has been demonstrated in both ambient air PM2.5 (diameter < 2.5 µm) and in PM from a variety of combustion sources. Thus, low-temperature, thermal treatment of soils can potentially increase the concentration of EPFR in areas in and around Superfund sites. In this review, we will outline the evidence to date supporting EPFR formation and its environmental significance. Furthermore, we will address the lack of methodologies for specifically addressing its risk assessment and challenges associated with regulating this new, emerging contaminant.

Environmentally persistent free radicals in PM2.5: a review

“…•− ), hydroxyl radical ( • OH), and hydrogen peroxide (H 2 O 2 ) [8]. An important means by which PM 2.5 can induce oxidative stress is through the redox cycling of environmentally persistent free radicals (EPFRs) [9]. It has been reported that one EPFR can generate approximately 10 hydroxyl radicals [10].…”

Formation of environmentally persistent free radicals (EPFRs) on ZnO at room temperature: Implications for the fundamental model of EPFR generation

“…The impact of particulate matter (PM) on human health has become a major environmental issue recognized by government agencies 1–3 . The toxicity of transition metal-containing PM is significantly increased by the formation of environmentally persistent free radicals (EPFRs) – composite metal oxide/organic radical complexes with lifetimes of hours or days under ambient conditions 3,4 .…”

“…The toxicity of transition metal-containing PM is significantly increased by the formation of environmentally persistent free radicals (EPFRs) – composite metal oxide/organic radical complexes with lifetimes of hours or days under ambient conditions 3,4 . EPFR-containing PM has been demonstrated to produce pulmonary and cardiovascular dysfunction in animal models, with greater toxicity than non-EPFR-containing PM 5–11 .…”

Fire toxicity – The elephant in the room?

“…The shape, size, and composition of particulates determine their toxicity [57,58]. Organic species are chemisorbed onto their surface through transition metal oxides [59,60,61]. This results in surface-stabilized, environmentally persistent free radicals (EPFR), which continue to exist in ambient air from hours to months, and can last at least 12 h in biological fluids.…”