Ultrafine particle emission and its potential health risk from residential solid fuel combustion

Abstract

Residential solid fuel combustion for domestic heating and cooking is an important source for ambient and indoor air pollution. It produces large amount of PM2.5 during the combustion process and introduce significant health risk to indoor personal. Recently many studies have indicated that ultrafine particles (PM0.1) contribute to majority of particle number concentration of the emitted PM2.5 from residential solid fuel combustion. However, the emission of ultrafine particles and their potential health impact are not drawing much attention since investigations on their emission and health risk assessment are usually merged into PM2.5 mass concentration based evaluations. Therefore, ultrafine particle emission from residential solid fuel combustion, as well as their potential health impact, may be overlooked in current PM emission control and management. This review summarizes the current research processes reported in literatures on ultrafine particle emission from residential solid fuel combustion, including their formation mechanism, emission characteristics, as well as the potential health impact from the related indoor ultrafine particle exposures. First of all, it can be concluded that ultrafine particles are mostly formed via chemical reaction, homogeneous nucleation, and heterogeneous nucleation of organic and inorganic vapors released during combustion processes due to the low combustion temperature and efficiency. The formed ultrafine particles will sequentially grow into larger particles via heterogeneous nucleation, coagulation and sintering. Meanwhile, as reported in most of the literatures, both laboratory combustion platform studies and indoor air monitoring studies have indicated that ultrafine particle emission is generally occurred during residential solid fuel combustion in regardless of fuel and stove types used as well as combustion modes. These ultrafine particles account for the majority of particle number concentration. In real residential households, ultrafine particles emitted from residential solid fuel combustion will elevate the indoor particle number concentration, which eventually introduce significant health risk to indoor personal. Although there are still debates on the general associations between ultrafine particles and certain health effects, it is clear that health effect induced by exposure to ultrafine particles should be carefully considered at least in residential solid fuel combustion aspect. Finally, the current issue of using PM2.5 mass concentration as evaluation parameter without considering number concentration of ultrafine particle is discussed. Since PM2.5 mass concentration is mostly dominated by larger particles, it may not effectively represent the characteristics of ultrafine particle emission as well as their health impact. Therefore, it seems that merging ultrafine particles into PM2.5 mass concentration will significantly overlook ultrafine particle emission and their health impact in residential solid fuel combustion. From PM emission control aspect, it can be concluded that although the current strategies based on PM2.5 emission control (i.e. using “cleaner” fuels and/or more advanced stoves) have significantly promoted the reduction of PM2.5 mass concentration, they may not be effective in controlling ultrafine particle number concentration emitted from residential solid fuel combustion. Therefore, we suggest that control and management procedures on ultrafine particles should also be considered (i.e. from particle number concentration aspect) in addition to the current PM2.5 mass concentration emission control and management in residential solid fuel combustion.