Stratification of particulate matter in a kitchen: A comparison of empirical to predicted concentrations and implications for cookstove emissions targets

Abstract

Household air pollution continues to be a global public health burden as 2.8 billion people still rely on open fires or inefficient cookstoves. The majority of morbidity and mortality are related to inhalation of fine particulate matter (PM), and many initiatives have focused on reducing PM emission and resulting exposure. For this reason, targets for emission rates to meet air quality guidelines have been set by the World Health Organization and International Organization for Standardization. These targets are based on a single-zone box model that assumes released particles are distributed uniformly throughout the kitchen. However, particulate matter is known to stratify vertically and therefore personal exposure to the cook and others in the kitchen may not be predicted accurately by a single-zone model. Therefore, this study compares empirical measurements of PM stratification to box model predictions to determine if the single-zone model is appropriate for predicting PM exposure at the expected breathing zone of the inhabitants. A test kitchen with a fixed ventilation rate and dimensions common to kitchens in rural areas was built and fitted with thirty HAPEx PM Sensors were evenly distributed at 4 different heights within the kitchen. Seventy modified water boiling tests were performed with a common rocket stove operating at known firepower and emissions rates in two testing phases at 10, 15 and 20 air exchanges per hour. Results showed stratification was less pronounced with increasing ventilation and with decreasing firepower. Measured concentrations at 15 ACH were compared to the predicted value from the single-zone model, showing equilibrium at a height of approximately 1.6 m. The box model over predicted concentration at heights lower than this, including a 23–63% excess at the 1.3 m high breathing zone of a typical standing cook. This implies that the current emission rate targets may be overly conservative under these conditions, precluding use of some types of biomass cookstoves that may actually be able to meet air quality guidelines. Future modeling and empirical studies should be conducted to understand under what conditions the use of the box model is appropriate, and when it should be modified for more accurate incorporation of the effects of stratification.