Incomplete burning of wood weakens air quality

Individual households can harm air quality of their nearby area by burning wood in an inefficient manner, according to a new study by the Finnish Institute for Health and Welfare (THL).

In the winter of 2020–2021 THL examined how concentrations of air pollutants varied temporally and regionally in a detached housing area favouring small-scale wood combustion in the city of Kuopio. The measurements were implemented using a mobile measurement cart on different days of the week and at different times of day.

“We measured high air pollution levels along the route, but they were generally short-lived. However, on evenings with cold weather and low wind speed, the levels stayed high all evening. In these situations, even the emissions from individual houses led to significant deterioration of air quality in the area”, said Tarja Yli-Tuomi, Senior Researcher at THL

Although the results of the study apply to the winter period, burning wood causes emissions and adverse health effects in other seasons as well, especially in densely populated areas and in calm wind conditions.

According to the study, outdoor activities in a quiet residential area with detached housing can expose people to levels of air pollution that are equal to a walk on a busy city street.

“Our measurements showed that in the area under study, the amount of black carbon released into the air in the combustion process could reach five micrograms per cubic metre of air. That is high, when we know that in 2020 the highest hourly level for black carbon at the air quality station on Mannerheimintie in Helsinki was 7 micrograms per cubic metre of air”, Yli-Tuomi added.

According to Mikko Sokura, Environmental Inspector for the City of Kuopio, the effect of small-scale burning of wood in 2021 was fairly evident at air quality monitoring stations in areas where single-family houses predominate in Kuopio and Siilijärvi.

“The small-scale burning of wood specifically raised the levels of harmful fine particles”, he added.

In order to minimise the release of emissions that are harmful for health and the climate, wood should be burned as cleanly and efficiently as possible. This is important especially in densely built residential areas where a fairly large number of people can be exposed to emissions from one house.

“To minimise harm, special attention should be paid to how a wood-burning sauna stove is heated, because emissions specifically from sauna stoves can easily reach high levels. Compared with heat-retaining fireplaces, fires in sauna stoves must give off intense short-term heat. However, the structure of sauna stoves is fairly simple, and because of their low efficiency, they consume large amounts of firewood compared with the heating effect. Learning a cleaner manner of burning also has economic benefits, because emissions constitute wasted energy”, Yli-Tuomi said.

The study also made a set of recommendations to reduce the harmful particle emissions.

The recommendations included taking the firewood indoors 1-2 days before using it, removing the ash and open the damper, making sure that the room is adequately ventilated, using only firewood that is clean and dry - no trash or wood treated with chemicals, using small pieces of wood, about 5 cm. in diameter, as kindling for the first load, with plenty of space between the pieces and adding only large pieces of wood with a diameter of about 30 cm. piled close to each other.

High levels of particles from the burning of wood can cause symptoms even from short periods of exposure. The most common symptoms include irritation and mild respiratory symptoms, such as irritation of the throat and eyes, runny nose, and cough.

Long-term exposure of several years or decades increases the risk of chronic heart, and respiratory illnesses. In addition, studies at the population level have linked high daily exposure to deaths from respiratory and cardiac illnesses.