Outdoor air pollution is a significant global health concern, causing millions of premature deaths annually. A major component of this pollution is volatile organic compounds (VOCs), which are precursors to ozone and particulate matter. Acute VOC exposure is linked to respiratory issues, while chronic exposure is associated with cardiovascular disease, neurological disorders, and preterm births. Certain VOCs, like benzene and formaldehyde, are known or probable human carcinogens. While numerous studies have examined VOCs at local, regional, and national levels, there's a lack of comprehensive global assessments. This study aims to fill this gap by utilizing a global chemistry-climate model to simulate VOC distributions and quantify the associated lifetime cancer risks from 2000 to 2019, highlighting spatial and temporal variations and health disparities across different income levels. Understanding the global burden of VOC-related cancers is crucial for developing effective mitigation strategies and addressing environmental injustice.

Previous research on VOC exposure and health impacts has largely focused on smaller geographical scales, such as individual cities, regions, or nations. Studies like Xiong et al. (2019) assessed VOC sources and risks in Canadian cities, finding that inhalation cancer risks in western Canada exceeded acceptable levels. Strum et al. (2019) examined cancer risks from hazardous air pollutants across the US, identifying formaldehyde and benzene as major contributors. Zhu et al. (2019) utilized satellite data to map formaldehyde concentrations and cancer risks in the US. Partha et al.'s work investigated BTEX-induced preterm births globally. However, these studies lacked a global perspective and a comprehensive temporal analysis, making it difficult to understand the global health burden of VOCs and associated disparities across regions and income groups. This study addresses these limitations by providing a comprehensive global analysis of VOC concentrations and cancer risks over two decades.

This study employed the Community Earth System Model (CESM) version 2.2.0 with Community Atmosphere Model with Chemistry, version 6 (CAM6-Chem), a three-dimensional chemistry-climate model, to simulate global surface NMVOC concentrations from 2000 to 2019. The model was run at a 1.25° longitude and 0.9° latitude resolution, using Modern-Era Retrospective analysis for Research and Applications, version 2 meteorological fields. Anthropogenic emission data were primarily from the Community Earth Data System (CEDS) release v2021_04_21, providing monthly emissions for eight sectors. Individual VOC emissions were estimated using scaling factors derived from the Coupled Model Intercomparison Project Phase 6 (CMIP6) inventory. Biomass burning emissions were from the CMIP6 inventory by van Marle et al. (2017). Model performance was evaluated by comparing modeled BTEX concentrations with in-situ observations from the US, Canada, and Europe. Health risk assessment involved selecting 10 hazardous VOCs for analysis, calculating population-weighted concentrations, and estimating lifetime inhalation cancer risk (LICR) for benzene, formaldehyde, and acetaldehyde, separated by age groups to account for varying susceptibility. Cancer burden (CB) was also calculated to assess population-wide risks. The study divided the globe into 11 regions to analyze spatial variations in exposure and risk.

Globally, NMVOC emissions increased by 10.2% from 2000 to 2019, with significant increases in Sub-Saharan Africa (+75.9%), the Rest of Asia (+44.0%), and China (+21.5%), but decreases in the US (-36.4%) and Western Europe (-44.8%). Energy and solvent sectors showed upward emission trends, while transportation and residential/commercial sectors declined. China consistently had the highest NMVOC concentrations (35.0-41.5 µg m³), followed by India and Latin America. Methanol was the most abundant hazardous VOC, while formaldehyde, acetaldehyde, and methyl ethyl ketone also had high concentrations. The integrated lifetime inhalation cancer risk (LICR) increased from 4.2 × 10⁻⁶ in 2000 to 4.4 × 10⁻⁶ in 2015 and 2019, with benzene contributing most to the risk. 36.4-39.7% of the global population was exposed to unacceptable LICR (>1.0 × 10⁻⁶) during the study period, with the highest proportions in China (80.3-84.3%) and India (49.4-55.8%). The global cancer burden from these three carcinogens increased from 0.60 million in 2000 to 0.85 million in 2019, with China and India having the largest burdens. High-income countries showed decreased cancer burdens, while low-to-middle-income countries experienced increases, largely due to population growth and VOC emission changes. Open agricultural burning significantly contributed to the increased risks in some regions. Analysis by income level showed that low-middle-income countries had significantly higher exposure risks than other income groups.

The findings highlight the alarmingly high proportion of the global population exposed to unacceptable levels of carcinogenic VOCs, especially in low-to-middle-income countries. The unequal distribution of risk underscores the need for targeted mitigation strategies. The decrease in cancer burden in high-income countries demonstrates the effectiveness of emission reduction efforts, particularly concerning benzene. However, the increase in formaldehyde-related burdens in some high-income countries highlights the complexities of air pollution control, including secondary formation of pollutants. The study's emphasis on age-specific risks underscores the vulnerability of children to VOC exposure. The impact of open agricultural burning and wildfires emphasizes the need for sustainable agricultural practices and improved wildfire management. The study's global perspective provides crucial information for policymakers to address environmental injustice and promote equitable access to clean air.

This study provides a comprehensive global assessment of VOC exposure and cancer risks over two decades, revealing a substantial global health burden and significant health disparities. The findings emphasize the urgent need for emission reduction strategies tailored to specific regions and pollutants, including addressing the impact of open agricultural burning and wildfires. Future research could explore the synergistic and antagonistic effects of VOC mixtures, incorporate other exposure pathways (dermal, ingestion), and improve model resolution for more accurate risk assessments, especially in data-scarce regions.

The study assumed additive effects of VOCs, which may not fully capture complex interactions. The analysis considered only a subset of hazardous VOCs and primarily focused on inhalation exposure, potentially underestimating the total cancer risk. Data limitations led to assumptions about intake rates for specific age groups in certain regions. Inherent uncertainties in emission inventories, model configurations, and simulations could influence risk estimates, particularly in regions with limited ground-based measurements. The coarse model resolution might not accurately represent exposure variations in areas with diverse landscapes.