Ultrafine particles and health: the next frontier in understanding air pollution hazards

Ambient air pollution is a major risk factor for morbidity and mortality worldwide. Among the pollutants, fine particulate matter (PM2.5)—particles ≤2.5 µm in aerodynamic diameter—is recognised to cause the largest health impacts. Indeed, in 2021, ambient PM2.5 was estimated to be responsible for 4.7 million deaths and 120 million disability-adjusted life years lost worldwide.1 PM2.5 adversely affects health as it penetrates deep into the lungs and can enter the bloodstream, leading to the development or exacerbation of respiratory and cardiovascular diseases.2 Studies have also linked ambient PM2.5 exposure to other health conditions such as the development of diabetes mellitus and adverse birth outcomes.2 In many cities, ambient PM2.5 can frequently exceed 35 µg/m³, a level considered unhealthy for sensitive groups by the World Health Organization.3 Indeed, 41% of cities with a minimum population of 50 000 experienced population-weighted annual average PM2.5 concentrations above 35 µg/m³ in 2019.4 There is growing early evidence suggesting that ultrafine particles (UFP, <0.1 µm in aerodynamic diameter) are the most hazardous component of PM2.5, as these can penetrate even deeper into the lungs, are retained longer and enter the bloodstream more readily.5 Moreover, UFPs have a higher surface area per unit mass, allowing them to carry higher amounts of adsorbed materials, resulting in potentially higher toxicity.6 In contrast to ambient …