Understanding how cookstove PM2.5 emissions rates affect a cook’s personal exposure to PM2.5 is an important step in designing a successful health based cookstove program. Exposure is a function of the amount of pollutant created by the stove, and by the rate that it is removed from the breathing zone of the cook. The single zone box model was used to derive the WHO emissions rates targets that meet the WHO indoor air quality guidelines. The model predicts that at an air exchange rate of 15/hr and a cooking time of 4 hours an average emissions rate of 1.75 mg/min is needed to achieve an average indoor air concentration of 35 ug/m3 over a 24 hr period. Doubling the air exchange rate to 30/hr would allow an emissions rate of 3.5 mg/min. If the air exchange rate outside the house in a gentle wind was 60/hr the model predicts that an emissions rate of 7mg/min would protect health, similar to vented stoves with chimneys. Cookstove project developers should keep this mathematical relationship in mind when deciding the emissions rate target that the stove needs to achieve for a community with kitchens of a given air exchange rate. Projects that can influence kitchen ventilation should similarly prepare.

Our 2019 article in Energy for Sustainable Development, “Stratification of particulate matter in a kitchen: A comparison of empirical to predicted concentrations and implications for cookstove emissions targets”, describes experiments that examined the accuracy of the box model. It was confirmed that the WHO model predicted measured concentrations at a height of 1.6 m above the floor, but it was determined that it overpredicted the measured concentration by 23 to 63% at a height of 1.3 meters, and between 83 and 151% at a height of 1 meter. A height of 1.3 m can be associated with the breathing position of a standing woman of average stature, and a height of 1 m can be associated with her sitting position. Using the correction from the paper the health protecting emissions rate at a height of 1.3 meters and an air exchange rate of 30/hr would be between 4.3 and 5.7 mg/min. At 1 meter the emissions rate could be between 6.4 and 8.7 mg/min. While these emissions rates are still low compared to rocket stoves on the market worldwide today, they are in line with conservative projections of the user experience of emerging rocket stove technologies such as the Jet-Flame, and certainly within the range of performance of pellet burning stoves. Bringing a rocket stove outside likely brings the personal exposure down to WHO levels.

This summer we will continue these investigations using the LEMS emissions hood, the Test Kitchen, and personal exposure measurements when cooking outdoors. What is the effect of wind speed on concentrations of PM 2.5? We’ll try to find out.