Air pollution remains one of the leading environmental threats to human health worldwide.¹ Despite intensified efforts to reduce air pollution and align with the World Health Organization air quality guidelines, recent Global Burden of Disease estimates rank air pollution (Level 2) as the second leading cause of premature mortality globally, just behind high blood pressure.^2,3 In the Polish context, annual pollutant concentrations averaged 17 µg/m³ for fine particulate matter less than or equal to 2.5 μm in diameter (PM_2.5), 26 µg/m³ for coarse particulate matter (PM₁₀), 25 µg/m³ for nitrogen dioxide (NO₂), and 45 µg/m³ for ozone (O₃), with Kraków, Katowice, and Łódź being the most polluted cities in Poland.⁴ Given further evidence of the public health burden in Poland due to air pollution, recent studies have demonstrated the progression of coronary disease in diabetic patients⁵ and an increased risk of hospital admission for both acute and chronic coronary syndromes,⁶ as well as for exacerbations of chronic obstructive pulmonary disease.⁷

However, while much of the available evidence focuses on single‑pollutant exposure models, the reality of environmental exposures is much more complex. In this issue of Polish Archives of Internal Medicine, Zhan et al⁸ analyze the effects of both single and combined ambient air pollutants on daily mortality, based on data from the Qingpu Area, Shanghai, China, covering the years 2013–2019. Using a time‑stratified case‑crossover design including over 22 000 nonaccidental deaths, the authors investigated the relationship between short‑term exposure to PM_2.5, NO₂, sulfur dioxide (SO₂), 8‑hour moving average concentrations for ozone (O₃-8h), and carbon monoxide (CO), and mortality outcomes. Exposure estimates were derived from 3 air quality monitoring stations in the study area. The authors further used conditional logistic regression, exposure‑response curve modeling, and weighted quantile sum (WQS) regression to estimate both single pollutant and combined mixture risks.

Among the key findings, a 10 μg/m³-increase in PM_2.5 (at lag day 04) was associated with a 0.9% increase in the odds of nonaccidental death (odds ratio [OR], 1.009; 95% CI, 1.002–1.017), while NO₂ (lag day 06) was associated with an OR of 1.027 (95% CI, 1.01–1.045). O₃ and SO₂ also demonstrated significant associations at lag days 03 and 07 respectively, with the latter showing the strongest association (OR, 1.051; 95% CI, 1.015–1.089). For respiratory mortality, the risks were even more pronounced, with SO₂ (lag day 07) associated with an OR of 1.137 (95% CI, 1.054–1.226), and NO₂ and CO showing similar lag‑dependent increases.

WQS regression was used to explore mixture effects, identifying PM_2.5 (weight, 0.56) and NO₂ (weight, 0.32) as the dominant contributors to nonaccidental mortality, and SO₂ (weight, 0.71) as the leading contributor to respiratory mortality. A nationwide analysis from Italy complements these findings, showing that short‑term exposure to PM₁₀, PM_2.5, and NO₂ was associated with increased mortality across several causes of death, including cardiovascular, respiratory, and nervous system conditions.⁹ Notably, a 10 μg/m³-increase in PM_2.5 (lag day 0–5) was linked to a 9.6% increase in mortality from nervous system diseases, while exposure to NO₂ was associated with a higher risk of both respiratory and metabolic deaths. These results add to the growing body of evidence pointing to the broad and multisystem health impacts of air pollution.

Another critical insight from this analysis is the evidence of an interaction between high temperatures and O₃. At higher temperatures (>75th percentile), the effect estimate for O₃ and cardiovascular mortality at lag day 04 was markedly increased (0.13% increase per 10 μg/m³; 95% CI, 0.02%–0.25%), as compared with medium or low temperatures (P = 0.001). This finding adds to the growing evidence that temperature can act as an effect modifier for air pollution‑related health outcomes. For instance, in a large study covering 620 cities across 36 countries, Stafoggia et al¹⁰ found that heat exacerbates the mortality risks associated with air pollution. Herein, an increase in PM₁₀ led to a 1.21% rise in mortality on very hot days (99th percentile temperature), as compared with 0.54% on cooler days. Likewise, high O₃ levels combined with extreme heat were linked to up to 12.5% increased mortality, highlighting synergistic effects between heat and air pollutants.¹⁰

Notable strengths of the study by Zhan et al⁸ include its large sample size, rigorous design and methods, as well as consideration of combined pollutant exposure and temperature interaction. The case‑crossover design inherently controls for time‑invariant confounding, while time‑stratification by weekday within a month accounts for seasonality and long‑term trends. The integration of WQS and stratified interaction models reflects a methodological shift toward real‑world complexity and away from the reductionist, single‑exposure paradigm.¹ However, some limitations should also be acknowledged. The reliance on fixed‑site monitoring data rather than personal- or residential‑level exposure introduces potential exposure misclassification. Additionally, information on behavioral confounders was not available. It would also have been of interest to consider the role of long‑term exposure as a potential modifier of acute effects, as individuals chronically exposed to higher air pollution may be more susceptible to short‑term fluctuations.

From a public health and policy perspective, this study highlights the urgent need to address multiple pollutants simultaneously, particularly PM_2.5, NO₂, and SO₂, and to incorporate heat‑adaptation strategies into environmental health planning. As China and many other countries face rising temperatures and ongoing urbanization, air quality interventions should not only target emissions from transport and industry but also consider climatic coexposures. However, in regions where the average annual temperature is markedly lower than in Shanghai (18.8 °C in 2024),¹¹ for example in Mainz, Germany or Kraków, Poland, the role of high temperatures in exacerbating the health effects of air pollution may vary substantially. This is especially relevant given that, in such climates, pollution levels tend to rise during the colder months.¹² Real‑time alerts that integrate air quality and heat risk indices may help to reduce health care burden during acute episodes of exacerbated air pollution or extreme heat.

In conclusion, the study by Zhan et al⁸ provides compelling evidence that short‑term exposure to ambient air pollution is associated with elevated mortality risk, and that this risk is exacerbated under high‑temperature conditions. As this research field increasingly highlights the importance of the exposome framework, studies like this bring us closer to a nuanced understanding of how complex environmental mixtures impact human health.¹³