Air pollution causes millions of premature deaths worldwide each year, with fine particulate matter (PM2.5) identified as a major culprit. In response, countries from Ireland to China have promoted low-smoke or smokeless fuels as a clean alternative to traditional bituminous coal, peat, and wood. However, the health risks of ultrafine particles (UFPs, PM₀.₁, <100 nm) produced by low-smoke fuels have long been overlooked.
Now, a new study led by researchers from the Institute of Earth Environment of the Chinese Academy of Sciences (IEECAS) and international collaborators has revealed that low-smoke solid fuels cause a two- to three-fold increase in the number of UFPs emitted, even as the total mass of particulate emissions has been reduced.
Furthermore, UFPs can penetrate deep into the respiratory tract, cross the blood-brain barrier, and induce severe pulmonary inflammation with prolonged lung retention, thus promoting far higher lung and alveolar deposition compared with larger particles. For this reason, the risk of low-smoke fuels is higher than that of traditional smoky fuels.
The findings were published in Nature Geoscience on March 2, challenging the air quality regulatory framework that currently prioritizes the total mass of particulates over the number and size of particles.
The research involved a comprehensive, multi-dimensional study focused on Ireland's Low Smoke Area, including Dublin and Galway. The study integrated laboratory combustion experiments, an eight-year (2016–2023) long-term field monitoring campaign, human lung deposition model analysis, and atmospheric chemical transport simulations (WRF-CMAQ). The team tested commercially available solid fuels in Ireland, including raw fuels (peat, wood, and smoky coal) and industrially processed low-smoke fuels (anthracite ovoids and torrefied biomass briquettes). The researchers systematically characterized how many particles were emitted at different sizes, how much pollution was released both by mass and particle count, and what the particles were made of.
The results show that while low-smoke fuels reduced the total mass of particulate emissions by 50–77% compared to raw fuels, the number of UFPs emitted was much higher: 3.1×10¹⁵ particles kg⁻¹ (for anthracite ovoids) and 2×10¹⁵ particles kg⁻¹ (for biomass briquettes), an amount two to three times higher than for raw fuels. UFPs were the dominant particle type emitted by the low-smoke fuels, with a modal size of 73–78 nm, the smallest among all tested fuels.
Additionally, UFP deposition efficiency rose sharply as particle size decreased. Specifically, the lung deposition efficiency of particles <10 nm reached 90%, with 10–25% depositing in the alveoli. In contrast, particles of 100–1000 nm had a lung deposition efficiency of only 10–25%, with up to 90% exhaled. Source apportionment analysis indicated that low-smoke fuels accounted for 65% of lung-deposited particles and 43% of alveolar-deposited particles—far exceeding the total contribution of smoky fuels (16–20%).
Furthermore, even with reduced direct emissions of toxic compounds such as polycyclic aromatic hydrocarbons (PAHs), low-smoke fuels contributed 37% and 39% of lung-deposited and alveolar-deposited PAHs, respectively, due to the high deposition efficiency of UFPs. The large surface area of UFPs emitted by low-smoke fuels also provides additional condensation sites for co-emitted toxic pollutants, potentially enhancing particle toxicity—a factor not captured by particulate mass or PAH mass alone.
A cross-regional comparison among Dublin, Beijing, and Shanghai highlighted the unique pollution characteristics of UFP emissions from low-smoke fuels. Unlike the large particles (180–550 nm) in Beijing and Shanghai—dominated by secondary aerosol formation during haze events—particulates in Dublin were primarily UFPs from residential low-smoke fuel combustion. This resulted in lung deposition efficiencies that were orders of magnitude higher than in the Chinese megacities, despite lower PM2.5 mass concentrations. The same was true for the number of UFPs present in Dublin's air.
Prof. LIN Chunshui at IEECAS noted that the findings reveal a critical flaw in current clean fuel policies and air quality management: the singular focus on reducing particulate mass has led to the unintended consequence of increased UFP emissions, which pose a significant but underrecognized health risk. This problem is currently unaddressed by global air quality policies.
The study recommends revising air quality standards to incorporate UFP concentrations, particle size distribution, and deposited surface area as core monitoring and evaluation indicators, complementing existing particulate mass metrics; accelerating the popularization of high-efficiency, clean combustion stoves (e.g., automatically fired systems) that emit fewer toxic UFPs (e.g., alkali salt-dominated particles with lower lung deposition efficiency), and establishing long-term UFP monitoring networks to support evidence-based policy-making.
This research was supported by the National Natural Science Foundation of China, the CAS Key Research Program of Frontier Sciences, and other funding sources.
Nature Geoscience