Background:
The World Health Organization has identified substantial gaps in air quality and health research, particularly the limited availability of fine particulate matter (PM?.?) data from low- and middle-income countries such as South Africa. Although the association between PM?.? exposure and adverse health outcomes, including cardiopulmonary disease and premature mortality, is well established, most epidemiological and toxicological evidence originates from high-income regions in Europe and North America. African cities remain underrepresented, especially regarding long-term monitoring and chemical characterisation of PM?.?. This air pollutant is a heterogeneous mixture of organic compounds, secondary inorganic aerosols, elemental carbon and trace metals, with composition varying spatially, seasonally and by emission source. Fine and ultrafine particles penetrate deep into the respiratory tract, where inhaled carcinogenic metals such as nickel (Ni), arsenic (As), and hexavalent chromium (Cr(VI)) can induce oxidative stress, inflammation and increased cancer risk. Despite their recognised toxicity, data on PM?.?-bound carcinogens and associated health risks remain scarce in southern Africa.

Objectives:
This study compared PM?.? concentrations and carcinogenic trace element risks across four South African sites (Pretoria, Thohoyandou, Mabopane and Bloemfontein) to identify dominant carcinogenic metals, assess spatial and seasonal variability and evaluate site-specific exposure-related health risks.

Methods:
24-hour PM?.? samples were collected using GilAir-5 personal air samplers fitted with PTFE membrane filters at sampling frequencies ranging from every third to every sixth day between 2017 and 2023, depending on site. Gravimetric analysis was conducted under controlled temperature and humidity conditions. Trace element concentrations were quantified using energy-dispersive X-ray fluorescence. Descriptive statistical analyses were performed using SAS version 9.4. Health risks were evaluated following the U.S. EPA Human Health Risk Assessment framework, estimating hazard quotients and lifetime excess cancer risks (CRs) for inhalation exposure. The yearly WHO PM?.? guideline (5 µg/m³) served as the reference concentration. The EPA cancer inhalation unit risk value of 2.4 × 10⁻⁴ for As and Ni; and 1.8 × 10⁻² for Cr(VI) was applied.

Results:
Mean PM?.? concentrations exceeded the yearly WHO guideline at all sites: Prettoria (23.2 µg/m³), Thohoyandou (11.0 µg/m³), Mabopane (10.0 µg/m³) and Bloemfontein (11.0 µg/m³). Concentrations were consistently higher during cold and dry seasons, reflecting increased domestic fuel combustion and reduced atmospheric dispersion. Ni emerged as the most consistent carcinogenic risk across multiple sites, with CRs ranging from 1.2 × 10?? to 3.51 × 10??, exceeding commonly accepted benchmark levels and indicating persistent exposure from sources such as traffic emissions, industrial activities, coal combustion and waste incineration. As posed an additional seasonal cancer risk in Pretoria, likely associated with biomass burning and domestic fuel use during colder months. In Bloemfontein, Cr(VI) was the dominant carcinogen (CR = 4.32 × 10??), suggesting chronic exposure from persistent, source-specific emissions.

Conclusions:
PM?.? pollution across South Africa remains well above the yearly WHO guideline, with carcinogenic trace metals contributing substantially to health risks. These findings underscore the need for stricter air quality standards, expanded chemical speciation monitoring and targeted emission control strategies, particularly to protect vulnerable populations such as children and communities residing near major pollution sources.

Map and PM2.5 sampling set up