Footprints of Airborne Ultrafine Particles on Urban Air Quality and Public Health

Emergence of airborne ultrafine particles (UFPs; those below 100 nm in diameter) in both indoor and outdoor urban settings is continuously attracting attention of the air quality science and management communities worldwide due to their probable adverse impacts on human health and the environment. A simple mean to visualise their tiny size is that the width of a single human hair can accommodate, in parallel, ~60,000 and 600 particles of 1 and 100 nm diameter, respectively. An exponential increase in number of UFP studies has been noticed during the last decade, suggesting a notable advancement in the area of their measurements, physico–chemical characterisation, dispersion modelling and exposure assessment [1]. However, this progress is not yet adequate enough to inform regulatory decision making on a particle number basis. The first question in this context can be raised that where do these UFPs come from and why might we need to control them? Further question can be asked: if the UFPs are needed to be controlled, what are the practical and technical challenges in envisaging control measures? This article revolves around these questions and makes an attempt to highlight recent advances in our understanding and future research priorities. The subsequent paragraphs focuses on the concerns related to outdoor UFPs in urban settings. For the completeness of the article, the last few sections very briefly introduce the UFP related issues in the context of indoor environments.