The Potential Air Quality Impacts of Prescribed Burning and Wildfires in the British Uplands

Smoke from uncontrolled wildfires contains far higher levels of harmful pollutants than the smoke from traditional prescribed burns. Yet, this crucial distinction is being overlooked in policy development.

The result is a rising risk of severe wildfires, increased exposure to dangerous particulate matter, and the potential for long-term public health consequences. We believe this issue deserves immediate and serious attention.

If current restrictions continue without adjustment or coordination, we risk not only losing a vital land management tool but also exposing communities and emergency services to preventable harm.

The Science is Clear

Wildfires release up to four times more fine particulate matter (PM₂.₅) than prescribed burning. These pollutants are directly linked to increased hospital admissions for asthma, chronic obstructive pulmonary disease, heart failure and other cardiopulmonary conditions.

Studies from the UK and abroad consistently show that wildfire smoke presents a far greater and more acute health risk than that produced by small, controlled burns. Yet, the current regulatory framework is discouraging land managers from carrying out safe, traditional winter burning.

The licensing scheme for vegetation burns over deep peat has become overly complex, delayed and heavily influenced by a philosophical opposition to burning. Land managers are increasingly walking away from a system that takes years to deliver answers - if it does at all. As a result, vital preventative work is not being done.

This policy environment is also eroding the traditional skills associated with controlled burning, skills identified in the first State of Nature (2013) report as important to rural heritage and biodiversity.

Once lost, these practices will be extremely difficult to recover. We are also concerned that emissions from traditional winter burns are now being used to discredit the practice, without acknowledging the far more severe emissions from uncontrolled wildfires. This selective framing risks misleading the public and undermines sound land management.

Government bodies, including Defra, Natural England, National Parks and National Landscapes, have a duty to present the full picture, especially where public health is at stake.

Health Impact Concerns

The incomplete burning of vegetation generates particulate matter (PM) and polycyclic aromatic hydrocarbons (PAHs) (Lee, 2010; Ramesh et al., 2011). Sustained exposure to high PAH concentrations is linked to serious health problems, such as several forms of cancer, cardiovascular disease and suppressed fetal development (Kim et al, 2013; Abdel-Shafy & Mansour, 2016).

Whereas sustained exposure to low concentrations of PM₂.₅ has been linked to cardiopulmonary disorders and impairments, the initiation and progression of diabetes mellitus and adverse birth outcomes (Feng et al., 2016).

PM and PAHs are certainly produced during prescribed heather burning. But we lack the evidence to determine whether they are produced in high enough concentrations, for long enough, and close enough to human settlements to negatively impact human health. However, there is evidence from several studies that provide us with some background information on this issue.

Evidence From the Scientific Literature

Graham et al. (2018): ‘Impact of the June 2018 Saddleworth Moor wildfires on air quality in northern England’ 

They monitored the air quality impacts of a large wildfire that ignited in June 2018 on Saddleworth Moor, UK. During the wildfire, surface concentrations of PM2.5 were 4 to 5.5 times higher than the seasonal average. Furthermore, carbon monoxide and carbon dioxide emissions were comparable to those emitted from a medium-sized power station (0.07–4.69 kg s⁻¹).

Harper et al. (2020): ‘Chemical composition of wildfire ash produced in contrasting ecosystems and its toxicity to Daphnia magna’ 

They examined the chemical composition of wildfire ash from several different habitats worldwide and tested its toxicity on the ecologically sensitive aquatic macroinvertebrate Daphnia magna. The results confirm that PAHs are indeed produced during the burning of vegetation, including heather. They also suggest that wildfire ash is not toxic to D. magna.

Rappold et al. (2011): ‘Peat bog wildfire smoke exposure in rural North Carolina is associated with cardiopulmonary emergency department visits assessed through syndromic surveillance’ 

They investigated a wildfire that occurred between the 1st of June and 14th of July 2008 within a peatland in North Carolina, USA. The relationship between the risk of suffering a range of health conditions was compared within and outside areas exposed to wildfire smoke. In the exposed areas, there were significant increases in the risk of developing asthma, chronic obstructive pulmonary disease, pneumonia and acute bronchitis. Emergency department visits associated with cardiopulmonary symptoms and heart failure were also higher within exposed areas.

Scott et al. (2012): ‘Fate and transport of polycyclic aromatic hydrocarbons in upland Irish headwater lake catchments’ 

They measured PAH concentrations within water and soil samples taken from five upland lake catchments in coastal regions around Ireland. The highest concentrations of PAHs in soil and water samples were from a catchment within an agricultural area where heather burning is common.

However, due to the presence of other PAH sources nearby (industrial emissions), the elevated PAHs concentrations cannot solely be attributed to prescribed heather burning. More importantly, this study cannot be used to infer the air quality and associated health impacts of prescribed burning because it did not measure post-fire atmospheric PAH concentrations next to the fire source nor within the human settlements nearby.

Tinling et al. (2016): ‘Repeating cardiopulmonary health effects in rural North Carolina population during a second large peat wildfire’ 

They investigated the cardiopulmonary health impacts of wildfire smoke exposure during a peatland wildfire in North Carolina, USA. The authors explored associations between daily atmospheric PM2.5 concentrations and cardiopulmonary emergency department visits during a 45-day period, with the first day being when the wildfire ignited. The study found that wildfire induced increases in PM2.5 was associated with an increased risk of cardiopulmonary conditions.

Vane et al. (2013): ‘Sedimentary transport and fate of polycyclic aromatic hydrocarbons (PAH) from managed burning of moorland vegetation on a blanket peat, South Yorkshire, UK’ 

They assessed the PAH concentrations within peat core, peat surface, stream water, moorland vegetation and reservoir core samples collected within the Broomhead Reservoir catchment, South Yorkshire, UK. While the study confirms that PAHs are produced during prescribed heather burning, the authors state that “No total or individual PAH concentrations exceeded the published sediment quality guidelines”.

This study cannot be used to infer the air quality and associated health impacts of prescribed burning because it did not measure post-fire atmospheric PAH concentrations next to the fire source nor within the human settlements nearby.

Summary of Wildfire Particulates and Air Quality Impacts

Vegetation-fire smoke contains a complex mixture of pollutants, primarily fine particulate matter (PM₂.₅), known to impair respiratory and cardiovascular functions (Borchers Arriagada et al., 2020; Johnston et al., 2012; Aguilera et al., 2021). These particles can enter the bloodstream and are often laced with toxic PAHs (Kim et al, 2013; Lee, 2010). 

In the 2019–20 Australian bushfires, PM₂.₅ levels exceeded 98 µg/m³, with estimates of over 400 excess deaths attributed to smoke exposure (Borchers Arriagada et al., 2020). Wildfire smoke exposure is consistently linked to increases in emergency hospital visits for asthma, COPD, and cardiovascular conditions (Rappold et al., 2011; Tinling et al., 2016).

Evidence also shows that wildfire-derived PM₂.₅ may be more harmful than urban sources (Aguilera et al., 2021). Controlled studies suggest wildfire PM₂.₅ causes greater respiratory inflammation and hospitalisation rates per unit increase than other pollution types. Globally, smoke from wildfires and agricultural burns causes hundreds of thousands of premature deaths annually (Johnston et al., 2012).

Conversely, smoke from prescribed burns tends to be lower in volume and pollutant concentration due to the controlled nature of the fires (Wade & Mobley, 2007; NWCG, 2020). Prescribed heather burning in the UK occurs in small patches during seasons with higher fuel moisture and greater atmospheric dispersion. Such burns avoid ignition of the moss and peat layers, which dramatically limits PM and PAH emissions compared to wildfires (Vane et al., 2013).

Multiple studies have quantified the difference in emissions: wildfires typically emit 2 to 4 times more PM₂.₅ per unit of biomass burned than prescribed burns, due to higher combustion of duff, coarse fuels, and prolonged smouldering phases (Urbanski, 2014; Prichard et al., 2020; Mitchell et al., 2014). This elevated emission factor, combined with the larger scale and duration of wildfires, results in significantly greater smoke-related health risks compared to prescribed fire practices.

Further Considerations in Relation to Traditional Winter Heather Burning 

Compared to wildfires, prescribed heather burning is much less severe, on a far smaller spatial and temporal scale, and generally occurs at a different time of year. As such, the air quality and human health impacts of prescribed burning are likely to be significantly lower than those recorded in the wildfire studies described above. 

The air quality and human health impacts of prescribed heather burning may also be lower than domestic wood burning. For example, unlike prescribed burning, domestic woodburning occurs daily in the colder months; it takes place at night when levels of smoke dispersion and atmospheric mixing height are lower (Wade & Mobley, 2007; NWCG, 2020); it produces smoke directly within areas of human habitation.

Finally, it is important that the negative impacts of prescribed heather burning are judged against the benefits it provides. Such benefits include providing habitat for ground-nesting birds and invertebrates, wildfire prevention and mitigation, and maintaining moorland plant communities.

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