Indoor and outdoor pollutants—No safe amount of air to breathe in
Authors: Ioana Agache, Marek Jutel, Cezmi A. Akdis
Journal: Allergy (2024)

Planetary health and human health are interconnected. Increased anthropogenic activity, including deforestation, urbanisation, modern agricultural and animal breeding practices, overuse of fertilizers, pesticides and antibiotic, production of synthetic chemicals and plastics, and the use of fossil fuels and biomass for energy, have altered the Earth's environment.1, 2 More than 350,000 chemicals have been introduced to human lives without any concern on their health impacts and more than 120,000 of them have been even inappropriately reported.3 99% of the global population breathe polluted air that exceeds the guidelines set by the World Health Organization (WHO). Globally, 7.8% of all deaths in 2019 were attributed to outdoor air pollution, with this fraction exceeding 12% in some countries. In 2019, there were 455,000 reported deaths attributed to asthma, predominantly occurring in low- and middle-income countries due to the difficulties associated with under-diagnosis and under-treatment. These countries also have a high prevalence of exposure to outdoor and indoor pollutants, including active or passive tobacco smoke.4-6 Asthma development and evolution follow a complex interplay between the individual genetic background and environmental exposures, such as indoor and outdoor air pollutants, infectious agents, allergens, irritants, or occupational factors.7-9 As physicians and scientists, we have seen first-hand how increasing outdoor and indoor air pollution impacts respiratory health, tried to understand the molecular changes driving the environmental-driven asthma symptoms and have used this knowledge to treat patients with asthma. The European Academy of Allergy and Clinical Immunology (EAACI) is the world leader in allergy and asthma science and education. The key opinion leaders of the environmental health world gathered around the EAACI initiative to evaluate the impact of outdoor and indoor pollution on patients with asthma and to provide recommendations for prevention, patient care and mitigation of the impact of short-term exposure to outdoor and indoor pollutants, including environmental tobacco smoke (ETS) and active smoking on asthma-related outcomes. The impact of extreme temperatures was also evaluated (Figure 1). Several systematic reviews (SR) were conducted.10-12 Multiple studies have shown an association between outdoor air pollutants and poor early life lung growth, development of allergic sensitization, acutely impaired lung function, respiratory tract infections, and numerous asthma-related outcomes, such as new-onset asthma, asthma exacerbations and deterioration of asthma control, lung function decline, increased use of asthma medication and of healthcare resources, including visits to emergency department and hospital admissions. However, epidemiological studies have often yielded inconsistent findings, and not all studies have found significant associations,13 which may be related to both variations in statistical, measurement, and modelling methodologies between studies as well as differences between countries and/or regions in the concentrations and composition of outdoor air pollution. The recent SR published by EAACI showed that following short-term exposure to outdoor pollutants referenced by the current WHO Air Quality Guideline as single or mixed pollutants (particulate matter [PM] 2.5, PM10, NO2, SO2, O3, CO, and heavy traffic) there is an increase in the risk of asthma-related adverse outcomes, especially for hospital admissions and emergency department visits.10 Unfortunately, there was low to very low certainty evidence that interventions aiming to reduce the level of outdoor pollutants may decrease asthma exacerbations. A large-scale human biomonitoring survey conducted in Europe between 2014 and 2021 identified at least 46 pesticides and their metabolites in adults and children across five European countries, with at least two pesticides detected in 84% of the samples collected. Exposure to outdoor pesticides such as fumigants may increase the risk of new-onset asthma in agricultural workers, while exposure to 1,3-dichloropropene may increase the risk of asthma-related emergency department visits.10 It has to be noted here that the focus of these EAACI guidelines was asthma-related outcomes and multiple effects of pesticides and pollution on other diseases remain to be further analysed. Global temperatures and the frequency and severity of climate change events such as heatwaves, cold spells, accompanied by wildfires, sand and dust storms, and thunderstorms have all increased over recent decades.1, 13 As per the SR conducted, heatwaves and cold spells may increase the likelihood of asthma-related emergency visits and hospital admissions, potentially contributing to higher asthma mortality rates.10 Among the environmental conditions, indoor factors are of particular interest because globally people spend more than 80% of their time indoors. Health effects following exposure to indoor air pollutants may be experienced immediately after exposure or years later. There is a clear impact of indoor air pollutants on the risk of new-onset asthma and numerous asthma-related outcomes, such as exacerbations and deterioration of asthma control, lung function decline, increased use of asthma medication and healthcare resources, including visits to emergency department and hospital admissions, and increased mortality. Ninety-four studies were included in the SRs evaluating the impact of indoor pollutants: volatile organic compounds (VOCs), cleaning agents, damp/mould and indoor pesticides. There is moderate certainty evidence that exposure to damp/mould increases the risk of new-onset wheeze.11 Exposure to cleaning agents may be associated with a higher risk of new-onset asthma and with asthma severity, while exposure to pesticides and VOCs may increase the risk of new-onset asthma.10 As the level of certainty is low or very low for most of the available evidence on the impact of indoor pollutants on asthma-related outcomes more rigorous research in the field is warranted. Tobacco consumption is the single largest avoidable health risk, and the most relevant cause of premature death in the European Union (EU), responsible for nearly 700,000 deaths every year. Around 50% of smokers die on average 14 years earlier. Despite considerable progress made in recent years with the tobacco-banning policies, the number of smokers in the EU is still high: 26% of the overall population and 29% of young Europeans aged 15–24 smoke.4-6 It has become evident that a considerable part of the increase in asthma prevalence worldwide is due to maternal smoking in pregnancy or due to early-life exposure of children to ETS acting through epigenetic transgenerational effects.14-17 The recently published SR of 67 longitudinal studies proved with moderate certainty evidence that prenatal ETS increases the risk of recurrent wheeze and may increase the risk of new-onset asthma and low lung function.12 Postnatal ETS increases the risk of new-onset asthma and recurrent wheezing and may impact lung function. Another SR of 24 longitudinal studies showed that ETS increases the risk of severe asthma exacerbations and impairs asthma control and lung function.12 Active smoking increases the risk of severe asthma exacerbations and suboptimal asthma control and may impact asthma-related quality-of-life and lung function.12 There is no high-quality evidence from the SRs on the causal relationship between exposure to pollutants and adverse asthma-related outcomes. This calls for better quality research to explore the causal association within the frame of unbiased exposomic research.2, 9, 18, 19 Increasing the quality of the evidence should involve the implementation of standardized methods for assessing and documenting both short- and long-term exposure, while distinguishing between household, occupational or environmental sources of pollutants. Nevertheless, this extensive research evidence indicates the need to avoid being exposed to severely polluted environments and to be prepared for any asthma unfavourable outcome in settings with heavy pollution. To support the healthcare professional caring for patients with asthma, EAACI prepared a Guideline on the impact of outdoor pollution exposure on asthma-related outcomes using the GRADE approach.20 Recommendations are provided in a framework supporting rational decisions for healthcare professionals and patients to individualize and improve asthma management and for policymakers and regulators as an evidence-informed reference to help setting legally binding standards and goals for outdoor air quality at international, national, and local levels. For clinicians, this evidence guides their asthma management strategies toward achieving optimal asthma control. It involves assessing exposure risks and impacts, customizing interventions based on the type of pollutant and considering co-exposures. Shared decision-making with the patient helps select the most suitable interventions. A personalized approach is also suggested when advising asthma patients about occupational exposure risks. Collaborating with patient representatives for raising awareness, educating, and advising patients—especially those belonging to the most susceptible and vulnerable groups—is also strongly recommended. Smart home or workplace-based asthma services that reduce or eliminate environmental asthma triggers, ingenious urban planning, green technology, and e-mobility incentives are all excellent tools to achieve optimal air quality. The current EAACI guidelines for asthma management are expected to significantly improve the efforts of healthcare professionals towards better asthma care. By offering patient-tailored solutions, these guidelines aim to enhance asthma-related outcomes. Multilevel, multi-sectoral adaptation, and mitigation actions are vital to reduce emissions, improve air quality, and provide safe housing. National, regional and international asthma programs should develop a strategic plan to document the outdoor and indoor pollution, implement adaptation and mitigation strategies and assess improvements. These programs must prioritize addressing environmentally-triggered asthma in underserved and disproportionately affected populations within their strategic plans and activities. Additionally, healthcare education programs at all levels should incorporate environmental health strategies. None. None. The authors declare no conflict of interest related to this manuscript. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.