Two new studies find that exposure to air pollution is inextricably linked with respiratory infections, revealing substantial health challenges. The researchers combined extensive data from health registries with modelled air pollution data and linked air pollution, inflammation in the body and respiratory infections. The results reveal complex mechanisms and show that even low-to-moderate pollution can substantially harm people’s health. The research emphasises the importance of continually focusing on the impact of air pollution and suggests the need for individualised health strategies based on genetic factors, even in countries with less air pollution.
Air pollution seriously threatens global public health, causing millions of deaths annually. However, the explanation for the effects of pollution is complex, and research within this field has been limited, especially large-scale studies in high-income countries with less pollution. Two new studies now elucidate associations between levels of air pollution and health risks – both the effects on systemic inflammation and the increased risk of respiratory infections.
“Our research is among the few large-scale studies done in countries with less air pollution. Many previous studies were performed in countries with high air pollution levels and large populations. Our research underscores the significant impact of air pollution on public health, even at low-to-moderate levels, explains Kathrine Kaspersen, Postdoctoral Fellow at the Department of Clinical Immunology, Aarhus University Hospital.
In one study, the researchers investigated how air pollution affects inflammation – the state of irritation that usually manifests as redness, swelling, heat and pain when the immune system encounters foreign organisms, such as in wounds. Inflammation can also arise as a result of exposure to small particles such as those in air pollution or tobacco smoke. The immune system responds by mobilising white blood cells and chemicals to protect the body.
“We can measure this through C-reactive protein (CRP), a common marker for systemic inflammation. The various components of air pollution we investigated are known to induce inflammation. High inflammation is associated with an increased risk of developing cardiovascular disease, and we have previously shown an association with infections,” says the first author, Bertram Dalskov Kjerulff, Research Assistant at the Department of Clinical Immunology, Aarhus University Hospital.
This cross-sectional study included 18,463 blood donors and linked their high-sensitivity CRP measurements to modelled air pollution data. The results were clear: exposure to various air pollution components was generally associated with higher CRP levels, even with low-to-moderate air pollution.
“Notably, the participants were blood donors and therefore otherwise healthy. This indicates that exposure to low-to-moderate air pollution, which is typical in Denmark, is sufficient to increase CRP,” explains Bertram Dalskov Kjerulff.
The higher the air pollution, the more inflammation
The study provides important insight into the biological mechanism linking air pollution with cardiovascular disease. Even low air pollution can strongly harm health through inflammatory processes.
“Note that we cannot conclusively determine that air pollution specifically caused the effect for each individual. We can only observe a correlation between high air pollution and increased levels of CRP,” adds Bertram Dalskov Kjerulff.
Exposure to various air pollution components was generally associated with 10–70% higher CRP, with a few being associated with lower CRP.
“Although this is very complex and we have difficulty in isolating simple causal relationships, the figures emphasise the overall trend. Inflammation increases as air pollution increases as measured using CRP,” says Bertram Dalskov Kjerulff.
Sea salt, ammonia and ozone
In the second study, the researchers focused on linking air pollution and the risk of respiratory infections among 3,653,490 adults aged 18–64 years in Denmark between 2004 and 2016. Air pollution was measured as the modelled average daily concentration of various components at each individual’s home address over three months.
“We included many air pollution components because we wanted to examine them individually. Two air pollution components that are important in a Danish context are NO2 and particulate matter smaller than 2.5 µm in diameter – PM2.5. We clearly found that exposure to both was associated with high risk,” explains Kathrine Kaspersen, first author.
For example, a 52% higher risk of hospitalization for respiratory infections was associated with high versus low NO2 exposure, and high versus low PM2.5 exposure increased the risk by 45%. Surprisingly, exposure to sea salt, ammonia and ozone was associated with reduced risk of respiratory infections.
“This may be because these pollutants typically have higher levels where other types of pollution are lower, such as outside the big towns, in the countryside and along the coast. This illustrates the importance of analysing air pollution holistically and considering local variation,” says Kathrine Kaspersen.
The researchers had great difficulty in correcting for population density in the new studies.
“Your infection risk is significantly affected by your daily interactions with other people and, consequently, your exposure to potential transmission. We have tried to correct for this by including the population of each municipality in our calculations,” adds Kathrine Kaspersen.
People do not have consistently high inflammation
Globally, the World Health Organization (WHO) estimates that air pollution causes 4.2 million premature deaths per year, and air pollution accounts for 17% of all deaths and illnesses from acute lower respiratory tract infections. According to the researchers, the new studies emphasise the importance of understanding how even low-to-moderate air pollution harms public health.
“Air pollution is relatively low in Denmark compared with many other countries, but it still harms people. Although Denmark’s situation is better than many others, we have not achieved an optimal state and can clearly improve,” explains Bertram Dalskov Kjerulff.
Air pollution affects not only the respiratory system but also inflammation and thereby biomarkers.
“Inflammation levels are not consistently high in individuals. Additional stressors, such as smoking, contribute to this inflammation. As a result, it accumulates, leading to increased stress and significant effects,” he says.
Denmark’s unique health registries
Ultimately, the studies therefore urge us to take air quality seriously and implement measures that can reduce exposure to air pollution, even in areas with less pollution.
“Air pollution cannot be avoided entirely, since we all live somewhere and have to breathe, but we can consider experiments to investigate how this harms health. Even countries that have reduced air pollution still have some. We need to understand how this affects our health and how to better protect ourselves from these effects,” explains Bertram Dalskov Kjerulff.
A strength of the study is the extensive access to Denmark’s health registries, which the researchers used to investigate how air pollution is linked to health.
“Denmark is unique in having exceptionally comprehensive registries that provide access to all diagnoses at hospitals. By analysing these records alongside modelled air pollution data, we can discern the timing of various diagnoses and explore the impact of air pollution on individuals based on their home address,” says Bertram Dalskov Kjerulff.
The street where you live
The excellent data open the door to further research and action to improve health given the constant threat from air pollution. The researchers will continue investigating additional information markers and genetic aspects.
“We can learn which mechanisms are involved in this constant relationship between low inflammation and low air pollution. If we understand more about individual risk factors, we can also develop more precise methods to assess how people react to air pollution,” explains Bertram Dalskov Kjerulff.
However, the research suggests that where people live and the types of air pollution are not the only factors that determine the health effects. Kathrine Kaspersen reflects on the potential implications of the research and shares her vision.
“In a broader context, these findings could be used to evaluate individual risk and provide personalised treatment, highlighting the significant role of the environment," suggests Kathrine Kaspersen, emphasising the need for more in-depth studies.
Further in the future
Both researchers think that this information therefore has clear potential in understanding the health risks associated with environmental factors and provides a basis for further studies to improve understanding of the relationships between air pollution, inflammation and health outcomes.
“This would facilitate more precise initiatives for individuals genetically predisposed to negative reactions to air pollution. The goal is to identify the sensitivity of each person to residing in an area with substantial air pollution,” says Bertram Dalskov Kjerulff.
Several research groups are striving to realise this dream of considering all these parameters in individual risk assessment.
“Implementation lies further ahead in the future. Ongoing work is needed to review limit values and reduce air pollution in Denmark. Periodic nationwide reports on the health impact of air pollution in Denmark already assess the risk of various diseases. Now, we can incorporate respiratory infections into this calculation,” concludes Kathrine Kaspersen.