Emission of Extremely High Concentrations of PM2.5 and Ultrafine Particles during Firewood Combustion

Abstract

Firewood still remains a primary cooking and space heating fuel in many rural homes in Bhutan, the combustion of which presents substantial indoor air quality problems for the occupants. Between 2012 and 2013 a team of experts from Bhutan and Australia undertook a major air quality study in rural areas in Bhutan. During the study we observed emission of extremely high concentrations of particles when stoves were operated for cooking/heating. While larger findings of the study have been published in several journals, so far, we have not reported the emission of episodic peak concentrations when fuelwood was burned. This is important since the short-term peak particle concentrations contribute substantially to the total exposure and the associated health risks. Therefore, the aim of this paper is to report extremely high concentrations of fine (PM2.5 <2.5 µm) and ultrafine particles (UFP <0.1 µm) observed during cooking/heating, when firewood was used as a fuel. Two sets of measurements were done (i) indoor fixed measurements during cooking and heating in two village homes and (ii) personal exposure monitoring for 59 village children, using real-time instruments DustTrak for PM2.5 and Nano Tracer for UFP. The mean UFP and PM2.5 concentrations (2.0×105 particles/cm3 and 1329µg/m3) observed during heating were 3 and 4 times higher than the mean concentrations measured before the heating. Likewise, the mean UFP and PM2.5 concentrations (6.8×105 particles/cm3 and 4429 µg/m3) during cooking were 64 and 69 times the mean concentrations measured before the cooking. For personal exposure monitoring, a mean UFP concentration of 1.1×105 particles/cm3wasobservedcorresponding to time spent by children in the kitchen during cooking. This activity contributed to 64% of the daily exposure, even though children spent only 9% the day in the kitchen while cooking. The results reveal (i) a substantial contribution of biomass fuels to indoor pollution levels, (ii) importance of treating peak concentrations separately when average exposure is computed from extended measurements.