The Rise of Biomass Combustion for Heating
Biomass burning is considered an increasingly important source of indoor and outdoor air pollutants worldwide. Due to competitive costs and climate change sustainability compared to fossil fuels, biomass combustion for residential heating is on the rise and expected to become the major source of primary particulate matter emissions over the next 5-15 years. Understanding the health effects and measures necessary to reduce biomass emissions of harmful compounds is crucial to protect public health.
The use of biomass for energy production is expected to double, while the use of coal, gas, and oil are all expected to decrease. In Europe, it has been estimated that small-scale domestic wood/biomass combustion will become the dominant source of fine primary particle air pollution by 2020, contributing 38% of total emissions. This shift towards biomass heating raises significant concerns about its impact on air quality and public health.
Characterizing Ultrafine Particles from Biomass Burning
Particles from biomass burning have a complex chemical composition, making it difficult to predict their health hazards based solely on ambient concentrations, particle size, and composition. Experimental data suggest that biomass smoke inhalation causes adverse lung effects that are influenced by the type of fuel and combustion phase. Since no striking differences in gas-phase components among different fuels under the same combustion phase were observed, particulate matter composition appears to be the key factor responsible for the varying adverse health outcomes.
Among the differently-sized aerosols, ultrafine particles (UFPs) – those less than 100 nm in diameter – have drawn increased attention in recent years due to the limited knowledge about their properties and impacts on human health. Owing to their small size, UFPs typically do not account for a significant portion of particulate matter mass but dominate in terms of number concentration. Their chemical composition and spatiotemporal distribution show high variability related to specific emission sources, distance from the source, and micrometeorology.
Despite the availability of advanced analytical techniques, accurately quantifying the chemical composition of UFPs remains a significant challenge due to the small mass associated with this particle fraction. The majority of toxicological studies on UFPs are still carried out by exposing cells to previously collected and characterized aerosol particles, which is a demanding task.
Sources and Characteristics of Residential UFP Emissions
Biomass combustion processes typically emit primary UFPs and gases that can promote ultrafine secondary particle formation. In addition to vehicle exhaust, UFPs in outdoor air can originate from a variety of sources such as industrial processes, energy production plants, aircraft and ship emissions, construction works, cooking, and domestic heating. An increase in the relative contribution from these non-traffic sources is expected due to the decline of tailpipe emissions from motor vehicles as a consequence of improved technologies and more stringent regulations.
Indoor air quality is also of great concern, as people spend the majority of their time in confined environments. Indoor UFPs comprise both primary and secondary aerosols emitted by various indoor sources, as well as outdoor aerosols that have infiltrated indoors. A general characterization of indoor UFP sources is difficult because many indoor emissions are strictly dependent on the type of activity under investigation.
Biomass Burning as a Major Source of UFPs
Receptor modeling approaches, such as Positive Matrix Factorization (PMF), have been used to retrieve the chemical profile of the biomass burning source in real-world conditions. These studies have provided valuable insights into the contribution of biomass burning to UFPs in ambient air.
- A study in Milan, Italy found that the biomass burning source was mainly related to a local (urban) contribution and accounted for 15% of the mass in the finest particle fraction (the Aitken mode). The ultrafine fraction gave the largest contributions to potassium, levoglucosan, and sulfate.
- In polluted California cities, wood burning was identified as the largest contributor (32-47%) to organic carbon in the ultrafine fraction during winter due to the widespread use of wood for domestic heating.
- A study in Sacramento, California found that the wood burning source was the largest contributor to the whole ultrafine fraction mass, accounting for up to 44%, and was the biggest source of organic carbon.
- During a severe wintertime air quality episode in California, wood smoke was the largest source of UFPs, contributing 3-43% to ultrafine particle mass and up to 60-80% during nighttime.
These studies highlight the significant impact of residential biomass combustion as a major source of UFPs in ambient air, especially during the winter heating season.
Composition and Toxicity of Biomass Burning UFPs
Laboratory-scale experiments have also provided insights into the composition and toxicity of UFPs generated from biomass combustion:
- UFPs from conventional masonry heater burning beech wood were dominated by potassium, sulfur, and zinc, with other contributors including carbon, calcium, iron, magnesium, chlorine, phosphorus, and sodium.
- UFPs from the combustion of wood logs were found to be dominated by total carbon, while potassium was significant in particles emitted by pellet burning. PAHs were more abundant in wood samples than in pellets.
- Anhydrosugars like levoglucosan were found only in wood stove UFP samples, not in pellet combustion, likely due to the lower temperatures in automatic appliances not favoring their formation.
- Compared to diesel particles, biomass UFPs had higher amounts of manganese and potassium but lower levels of metals and PAHs.
These compositional differences can lead to varying biological responses and toxicity. In vitro studies have demonstrated that:
- UFPs from wood combustion can induce pro-inflammatory and genotoxic effects, with UFPs from log wood combustion being more biologically reactive than those from pellet combustion.
- The inflammatory effects were linked to the presence of levoglucosan, while genotoxicity was related to elements like aluminum, iron, and polycyclic aromatic hydrocarbons.
- UFPs from fir log wood combustion caused the most significant increase in DNA damage compared to beech wood or pellets, likely due to the higher polycyclic aromatic hydrocarbon fraction.
These findings highlight the importance of fuel type and combustion conditions in determining the toxicity of biomass burning-derived UFPs.
Health Impacts of Biomass Burning UFPs
The small size and high surface area of UFPs make them potentially more harmful than larger particulate matter fractions. Their large specific surface area facilitates the absorption of toxic components, and their small size promotes their transport to different regions of the body.
Exposure to biomass burning UFPs can lead to a range of adverse health effects, including:
- Oxidative stress and inflammation in the lungs, which can spill over into the systemic circulation and contribute to cardiovascular and reproductive effects.
- Impairment of alveolar macrophage function, reducing the body’s ability to clear pathogens and increasing the risk of respiratory tract infections.
- Stimulation of innate lymphoid cells to produce cytokines that can induce airway hyperresponsiveness and asthma in genetically predisposed individuals.
- Genotoxicity and DNA damage, providing a biological rationale for the increased risk of lung cancer associated with exposure to particulate matter.
The World Health Organization estimates that household air pollution, mainly attributable to biomass combustion, accounts for approximately 3.8 million deaths per year, with an additional 4.1 million from outdoor air pollution. In Europe, the current contribution of biomass smoke to premature mortality amounts to at least 40,000 deaths per year.
Strategies for Improving Efficiency and Reducing Emissions
Despite decades of efforts to promote cleaner cooking and heating technologies based on biomass fuels, the displacement of polluting technologies has progressed slowly. Biomass combustion emissions are increasing in developed countries and are expected to become the major source of primary particulate matter emission over the next 5-15 years.
There is increasing evidence that wood log stoves generate particles that are more biologically active compared to pellet stoves, and that different wood types can produce particles with varying composition and biological activity. A strategic area of health research is the development and use of clean combustion approaches and emission control technologies, along with proper education of users.
A shift to clean fuel alternatives is desirable to reduce exposure to biomass combustion products. Further studies are needed to clarify the role of combustion conditions and biomass fuels in particle size distribution and composition, and to understand the molecular mechanisms behind the harmful effects observed.
Detailed physicochemical characterization of UFPs from biomass burning, identification of possible biological effects and underlying mechanisms, and improved source apportionment of UFPs in indoor and outdoor air are all crucial steps to improve air quality and implement targeted strategies for the abatement of this particle fraction, leading to healthier environments.
