Now well into October, the chill in the air most evenings harkens the arrival of wood burning season in the northern hemisphere. An open fire or wood burning stove keeps a house warm, drives out the damp, and is a cozy addition to the home during long autumn and winter evenings. However, for the vast majority of people in the USA this is a time-honored tradition and a supplement to primary sources of heat rather than a necessity for surviving the winter – less than 1.5% of homes in the USA rely on wood fire for heat (1).
Worldwide, the figure increases significantly. About a third of the global population – or around 2.4 billion people – rely on open fires or inefficient stoves for heating and cooking, year-round (2). Apart from the risks associated with open fire in the household and around children, this is a significant contributor to indoor pollution. Indoor pollution is linked to numerous adverse outcomes including COPD, lung cancer, asthma, and respiratory infections, disproportionately affecting populations in low- and middle-income countries (3). In 2020, it was estimated that around 3.2 million deaths per year can be attributed to the indoor air pollution created by these fires (2). When considering the contribution to ambient (or outdoor) pollution, the figure rises to 6.7 million deaths annually (2).
Of particular concern is the particulate matter that is generated during the combustion process. Nearly two-thirds of the overall annual deaths attributed to pollution (about 4 million) can be attributed to fine particulate matter, or PM2.5* (4).
Even in high-income countries, where pollution-related mortality has been steadily dropping since the dawn of the 21st century (3), pollution related to wood burning has been identified as an area for improvement. Although more efficient wood burnings stoves are used in the UK, a recent study by the British government found that PM2.5 produced from domestic combustion (burning of wood or coal in the home) is a significant contributor to particulate pollution in the United Kingdom (5). This has led to changes in regulation that have been rolled out over the past two years – the ban on selling and burning green or wet firewood (and coal) (6).
Wood that is green or wet comes from trees that have recently been felled. It is quicker to source and as a result cheaper to buy. Dry wood is more costly to purchase because the process to obtain it is more involved – it has either been kiln dried or seasoned, a process of natural drying in a ventilated space that takes up to a year. It would appear that green wood would be the obvious choice economically for purchase and burning, and thus the policy would be ill-adapted to address the needs of the population – many of whom are seeking more affordable options to rising fuel prices (7). However, as with many practices that impact the environment and human health, the problem comes down to a matter of efficiency.
Burning green wood is extremely inefficient. It does not hold fire well, creates excess ash (and thus contributes to a more involved burner maintenance process), and does not produce heat in the same manner as dry wood, which leads to a higher consumption of green or wet wood. It is estimated that for every pound of dry wood burned, it generates 1500 more BTUs** than a pound of wet wood, a difference of about 30% (8). From a public health and environmental perspective, the benefits of more efficient dry wood combustion are clear – it is expected that cutting out green or wet wood will have the effect of cutting PM2.5 and other emissions generated from domestic combustion in the UK by 50% annually (6).
For the consumer, there is financial incentive to use only dry wood as well. While dry wood tends to be more expensive at the point of purchase, the difference in efficiency means that for every 7 units of dry wood burned, 10 equivalent units of wet wood would be consumed. Thus, barring a dramatic difference in price, dry wood is far more economical to burn. For those with space to naturally dry wood, the consumer can capitalize on greater savings and further reduce their environmental footprint by purchasing green wood and seasoning it throughout the year, eliminating the need for kiln drying.
Broader social and economic conditions will dictate what type of wood fuel is accessible to people worldwide. However, for those with wood fuel options, choosing dry wood always comes with the greatest benefits. It is less costly, more efficient, and better for the environment and human health both inside and outside the home. And if purchasing wood, choosing locally sourced wood further mitigates the environmental impact of the process – it cuts down on the risk of the transmission of novel pests and pathogens that will affect the live stands of trees in the area.
- Mark Anderson
*A brief definition of particulate matter pollution may be found here: https://www.eea.europa.eu/help/faq/what-is-particulate-matter-and
A more in-depth overview may be found here:
**BTU – British thermal unit
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1 – Uteuova, A. (2023). ‘Wood banks emerge as vital source of heat while US gas bills still on the rise’, The Guardian, 20 Jan [Online]. Available at : https://www.theguardian.com/us-news/2023/jan/20/us-gas-bills-wood-banks-heat-source#:~:text=Roughly%20half%20of%20US%20homes,to%2022%25%20in%20rural%20areas.
2 – World Health Organization (WHO) (2022). Household air pollution. WHO Fact Sheets [Online]. Available at: https://www.who.int/news-room/fact-sheets/detail/household-air-pollution-and-health.
3 – Lee, K. et al. (2020). ‘Adverse health effects associated with household air pollution: a systematic review, meta-analysis, and burden estimation study’, The Lancet Global Health, 8(11), pp. 1427-1434. [Online]. DOI: https://doi.org/10.1016/S2214-109X(20)30343-0.
4 – Thangavel, P., Park, D., and Lee, Y. (2022). ‘Recent insights into particulate matter (PM2.5)-mediated toxicity in humans: An overview’, Int. J. Environ. Res. Public Health, 19(22), 7511, MDPI [Online]. DOI: https://doi.org/10.3390/ijerph19127511.
5 – Department for Environment, Food & Rural Affairs (DEFRA) (2023). Emissions of air pollutants in the UK – Particulate matter (PM10 and PM2.5). Available at: https://www.gov.uk/government/statistics/emissions-of-air-pollutants/emissions-of-air-pollutants-in-the-uk-particulate-matter-pm10-and-pm25.
6 – DEFRA (2021). Restrictions on sale of coal and wet wood for home burning begin. Available at: https://www.gov.uk/government/news/restrictions-on-sale-of-coal-and-wet-wood-for-home-burning-begin.
7 – Sommerlad, J. (2023). ‘What are the new rules on log burners and are they going to be banned?’, The Independent, 06 Feb [Online]. Available at: https://www.independent.co.uk/climate-change/news/log-burners-rules-wood-stoves-b2276589.html.
8 – Bartok, J. (2003). Heating with wood and coal. Revised edn. Ithaca, New York: Natural Resource, Agriculture, and Engineering Service (NRAES) Cooperative Extension. Available at: https://ecommons.cornell.edu/server/api/core/bitstreams/c9e3f7fe-5a18-4170-8e56-422869e0183d/content.
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