group of doctors





Air pollution has both acute and chronic effects on respiratory health, affecting a number of different systems and organs.

Air pollutants such as nitrogen oxides, ozone, sulphur dioxide, carbon monoxide, and particulate matter affect different parts of the respiratory tract via a variety of mechanisms resulting in respiratory symptoms.

Air quality affects everyone, but some people are more at risk than others. Children and older adults, individuals with pre-existing cardiovascular or respiratory diseases, or genetic polymorphisms are at increased risk of air pollution-related health effects. This can be exacerbated when our body’s defence mechanisms are impaired.

Research is ongoing to identify causal associations of air pollutants with adverse health outcomes, and to uncover plausible biological mechanisms for these effects.

Here, The Clean Breathing Institute discusses the risk factors associated with air pollutants and the respiratory health symptoms associated with air pollution in those individuals who are at increased risk of adverse effects.

man profile

How does air pollution affect respiratory health?

Respiratory health effects range from minor upper respiratory irritation to chronic respiratory and heart disease, lung cancer and acute respiratory infections; air pollution can also aggravate pre-existing heart and lung disease.1–6 Short- and long-term exposures have been linked with premature mortality and reduced life expectancy.1

Pollutants with the strongest evidence for adverse effects on respiratory health include particulate matter, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide. Particulate matter is used as an indicator of air pollution and is usually divided into groups based on particle size.1,3–5 Coarse particulate matter has an aerodynamic diameter of between 10 μm and 2.5 μm, and is referred to as PM10–2.5.6 Fine particulate matter has an aerodynamic diameter of less than 2.5 μm, is referred to as PM2.5 and can penetrate deep into the lung.5,6 (Figure 1).


diagram - air pollution

Figure 1. Relative size of particulate matter that can enter the respiratory tract.3,4,6–9 PM, particulate matter. Note PM10-2.5 is represented as 10 μm particles and PM2.5 as 2.5 μm particles shown to scale relative to a grain of silica sand.




Air pollutants including particulate matter of different sizes affect different parts of the respiratory tract resulting in respiratory symptoms and changes in lung function (Figure 2).

The association of fine particulate matter (PM2.5) – which can reach deep into the lungs – with cardiorespiratory disease and mortality is well documented.10 Ultrafine particles (<0.1 μm) can enter the bloodstream and travel to distal organs, and are of considerable concern. No safe level of small particulate pollution has been identified.11




Particulate matter


Figure 2. Particulate matter in air pollution, respiratory symptoms and health effects. CNS, central nervous system; PM, particulate matter.1,6–9,12–21 



What are the symptoms of air pollution exposure associated with respiratory health?

All types of air pollution, at high concentration, can affect the airways and similar respiratory effects are also observed with long-term exposure to lower pollutant concentrations.1 The most common upper respiratory tract symptoms reported after exposure to air pollution include nose and throat symptoms. These symptoms include non-allergic rhinitis and nasal mucosal erythema, sinusitis, nasal itching, runny nose, nasal congestion, sneezing, dry mouth and throat, productive cough and dry cough, wheezing and dyspnoea.14,15,22,23

Symptoms such as nose and throat irritation, followed by bronchoconstriction and dyspnoea, especially in asthmatic individuals, have been reported after exposure to increased levels of sulphur dioxide24 and nitrogen oxides25. Cough and dry cough, sneezing and runny/stuffed nose have been associated with exposure to PM2.5 and nitrogen dioxide during the first year of life.26,27

Particulate matter that penetrates the lung epithelium can initiate lung inflammation,28 worsening pre-existing lung conditions and exacerbating symptoms of asthma and chronic obstructive pulmonary disease.

Air pollutants such as nitrogen oxides can increase the susceptibility to respiratory infections.29,30 Significant associations have been found between the number of outpatient consultations due to upper respiratory tract infections in Hong Kong and the concentrations of the air pollutants PM10, nitrogen dioxide, ozone, and sulphur dioxide.30

Indoor air pollution and respiratory symptoms

Indoor air pollution in buildings with high levels of airborne particles and gaseous pollutants can lead to complaints of upper respiratory symptoms, such as upper respiratory tract irritation and infection.15,31 These can include nose itching, runny nose, nasal congestion, sneezing, and dry throat, and are linked to indoor carbon dioxide levels and volatile organic compounds.15 Non-allergic rhinitis symptoms have been associated with exposure to particulate matter such as dust, cleaning solvents and strong odours, and are exacerbated by temperature and humidity changes.23

It is important to note that symptoms are not a reliable indicator of whether air pollution is at a level that poses a risk to health. There may be no immediate symptoms, even at relatively high concentrations of exposure.




Air quality affects everyone, but some people are more at risk than others.




Babies: Exposure to air pollution during pregnancy is associated with low birth weight, pre-term birth and increased risk of developing asthma, rhinitis and eczema.1,32–36


Children: Children living in polluted areas are more likely to suffer from coughs, wheezing, asthma and impaired lung function.37–40

Older individuals

Older individuals: Older people may be at risk due to reduced lung function that occurs with ageing and the presence of co-morbid pulmonary and cardiovascular conditions.40,41

Older individuals

People with pre-existing conditions: Individuals with conditions such as asthma, chronic obstructive pulmonary disease or heart disease may be at risk.1,40,42

Prenatal exposure to air pollution

There is mounting evidence for the adverse effects of prenatal air pollution on lung development, respiratory health and development of chronic disease in adulthood.43 Air pollution may also influence pregnancy by inducing systemic inflammation and oxidative stress, and could cross the placenta, and increase maternal susceptibility to infections.44 These effects could impair foetal growth and birth outcomes. BreatheLife – a joint campaign led by the World Health Organization, United Nations Environment, and Climate & Clean Air Coalition – provides tips on reducing prenatal exposure.45


boy with mask

Exposure of children to air pollution

Children are particularly vulnerable to air pollutants because their immune and antioxidant defence mechanisms are still developing and they have a faster breathing rate, taking in more air per unit body weight than adults, resulting in inhalation of higher doses of air pollutants compared with adults. Also, they generally spend more time playing outside, where they may be exposed to pollutants.46

Exposure of healthy individuals to air pollution

Some healthy people are more sensitive to the health effects of air pollution. They may experience health effects at lower air pollution levels than the average person even though they have none of the risk factors noted above. There may be a genetic basis for this increased sensitivity.47,48

boy with mask

How are particles deposited in the respiratory system?

Particle size, water solubility and chemical reactivity all determine where particular air pollutants impact the respiratory tract.14 Highly water-soluble and reactive irritants, such as sulphur dioxide and aldehydes, readily dissolve into the water around the nasal mucosa and initiate an inflammatory response, consequently having greatest impact in the upper respiratory tract.14 Ozone, having medium water solubility, can reach the trachea and bronchi, whereas low solubility gases, such as nitrogen dioxide, are also able to evade the defensive properties of the respiratory tract mucosa and reach the bronchioles and alveoli.14

Particulate matter may be deposited in three different regions of the respiratory system:

  • Extrathoracic (nasal, pharyngeal and laryngeal passages)
  • Tracheobronchial
  • Alveolar (pulmonary)

The key factors affecting deposition include: mode of breathing (mouth, nose or oronasal), breathing pattern (tidal volume, breathing frequency) and particle characteristics (size, shape, mass, hygroscopicity and solubility).8


What are the respiratory tracts defence mechanisms against particulate matter?

The upper respiratory tract comprises the nose, nasal cavity, mouth, throat (pharynx), and voice box (larynx). It plays an important role in monitoring the quality of inspired air as well as air warming and humidifying inspired air,49 and is the first line of defence against air pollutants.14

Different mechanical filters located in the nasopharyngeal (nose hairs and the turbulence in the nasal turbinates), tracheobronchial and alveolar regions protect the airways against inhaled foreign particles.50

The nose acts as the first in a series of protective mechanisms. With its narrow air passages, mucosal folds, and mucous layer covering ciliated epithelial cells, the nose can effectively filter most coarse particles.

Inhaled fine particle pollution deposited on the surface of the airways is cleared by two mechanisms:

  • Mucociliary clearance 
  • Phagocytosis

Mucociliary clearance

Mucociliary clearance is a vital self-clearing process of the airways and removes the vast majority of inhaled particles deposited in the tracheobronchial airways.51,52 On average, 2 litres/day of nasal mucus is produced by the respiratory mucosa to trap particles and moisten airways.53 Mucociliary clearance utilises unique properties of mucus and the specific action of cilia to trap and remove hazardous components of inhaled air, such as some pollutants, bacteria and viruses.22 The cilia beat in a synchronised manner, moving the mucus as well as substances trapped within the mucus, out of the nose so they can be swallowed or expectorated.14,22,54,55

Mucociliary transport in healthy individuals clears most insoluble particles within 24 hours of deposition under normal conditions.56 In chronic conditions such as respiratory diseases like asthma and chronic obstructive pulmonary disease, or in smokers, normal clearance may be impaired and particle retention increases.14,22,54,55

Mucociliary clearance can also be impaired when air pollutants damage the respiratory lining and induce temporary dysfunction of ciliary movement.50 This can result in symptoms such as productive cough and dyspnoea.22 Particles that escape the mucociliary defence mechanisms can enter the lower airways and alveolar region.50


Phagocytosis by macrophages is the primary clearance mechanism for removing any foreign material such as particles or microorganisms from the alveolar region.54

Following exposure to particulate matter, alveolar macrophages secrete an array of pro-inflammatory mediators, leading to apoptosis and induction of local and systemic inflammatory responses.57 These macrophages were found to interact with airway epithelial cells to augment the production of a variety of chemokines and cytokines, leading to inflammatory airway responses.58

The efficiency of phagocytosis decreases with decreasing particle size below about 1 μm.59 Ultrafine particles (<0.1 μm) may be deposited in the alveoli and subsequently absorbed into the bloodstream.5,13

Repeated, frequent exposure to particle pollution may overburden the pulmonary defence system.


How do air pollutants exert adverse effects on the respiratory system?

The mechanisms through which air pollutants exert adverse effects include:

  • Induction of oxidative stress via the generation of reactive oxygen species, leading to airway inflammation5,60,61
  • Influencing immune function, such as the development of allergic disease and response to infections5
  • Increasing the risk of upper respiratory tract infections29,30 and symptoms such as rhinitis, sinusitis, sneezing and coughing14,22

Oxidative stress mechanisms and effects on the respiratory health

Combustion-derived particulates can generate free radicals and are highly oxidising.5 Particulate matter and gaseous pollutants appear to induce an inflammatory cytokine response in macrophages via the nuclear factor-κB pathway62 (Figure 3). Exposure to PM2.5 has shown an association with nasal inflammatory cells and cytokine mediators in children,60 while in nasal lavage samples, exposure to ozone combined with heat has been associated with the presence of CC16, which is a marker of nasal epithelial damage.63 Ozone has been associated with persistent structural airway and lung tissue damage.64

table - air pollution

Figure 3. Model of oxidative stress mechanisms and effects on the airway. AP-1, activator protein-1; CHO, carbohydrate; DEP, diesel exhaust particles; ecGSHpx, extracellular glutathione peroxidise; ecSOD, extracellular superoxide dismutase; GSH, glutathione; NF-κB, nuclear factor-κB; NO2, nitrogen dioxide; O3, ozone; PM, particulate matter; ROS, reactive oxygen species.65 This material has not been reviewed prior to release; therefore the European Respiratory Society may not be responsible for any errors, omissions or inaccuracies, or for any consequences arising there from, in the content. Reproduced with permission of the © ERS 2018. European Respiratory Journal Jan 2008, 31 (1) 179-197; DOI: 10.1183/09031936.00128106.



Air pollutants influence immune function and affect respiratory health

Diesel exhaust particles (DEPs) can modify the immune response and airway inflammatory processes (Figure 4). Early life exposure to high levels of DEPs in children sensitised to house dust mites by age 4 doubled the risk of developing asthma by age 7 compared with low DEP exposure.66 A systematic review and meta-analysis showed significant associations for black carbon, nitrogen dioxide, PM2.5 and PM10 exposures and risk of asthma development in childhood.67 Air pollutants can also both negatively affect other aeroallergens, such as pollen,68 and enhance allergic response.5,69 Pollen in areas with heavy air pollution expresses greater amounts of allergenic proteins compared with areas with less air pollution.68 Residual oil fly ash has been shown to enhance allergen-induced pulmonary allergic response in mouse models of asthma, suggesting that this type of particulate matter can induce/amplify allergic responses.5 DEPs can act as mucosal adjuvants in enhancing the immunoglobulin E response to allergens and skew cytokine production to a Th2 pattern, amplifying the allergic responses.5 In a randomised and blinded controlled human crossover exposure study, specific allergen-induced proteins in the lung were significantly enhanced with DEP plus allergen compared with either DEP or allergen alone.69 

diagram - air pollution

Figure 4 . Mechanistic model of the exposure–disease relationship illustrating a sequential event originating from the exposure to the development of asthma.5 AhR, aryl hydrocarbon receptor; DC, dendritic cell; IgE, immunoglobulin E; iLC, innate lymphoid cells; PAH, polycyclic aromatic hydrocarbon; PM, particulate matter; Th, T helper; VOC, volatile organic compound. Reused/adapted with permission from AME Publishing Company



Air pollutants increase the risk of upper respiratory tract infections

Air pollutants such as nitrogen oxides can increase the susceptibility to respiratory infections.29,30,70 In a study conducted on children in Taiwan, the presence of nitrogen dioxide and ozone peaks lasting for 6 days was significantly associated with increased risk of outpatient visits for acute upper respiratory infections.29



How to manage the health effects of air pollution

Though the respiratory system has remarkable resilience to air pollution via its repeated mobilisation of defence and repair mechanisms, constant exposure to elevated particle pollution will contribute to reduced respiratory function, even in apparently healthy people. Therefore, although exposure to air pollution is difficult to avoid completely, advising patients to take simple steps to reduce exposure may reduce the severity of lung and systemic adverse health effects in both healthy and more sensitive people.

Download our literature review and our advisory meeting summary to find out more about the risk factors associated with air pollution and the effects of air pollution on respiratory health.


    01. Kampa M, Castanas E. Human health effects of air pollution. Environ Poll 2008;151:362–7.

    02. Mannucci PM, et al. Effects on health of air pollution: a narrative review. Intern Emerg Med 2015;10:657–62.

    03. D'Amato G, et al. Urban air pollution and climate change as environmental risk factors of respiratory allergy: an update. J Investig Allergol Clin Immunol 2010;20:95–102; quiz following 102.

    04. D'Amato G, et al. Climate change and air pollution: effects on respiratory allergy. Allergy Asthma Immunol Res 2016;8(5):391–5.

    05. Huang SK, et al. Mechanistic impact of outdoor air pollution on asthma and allergic diseases. J Thorac Dis 2015;7:23–33.

    06. Araujo JA, Nel AE. Particulate matter and atherosclerosis: role of particle size, composition and oxidative stress. Part Fibre Toxicol 2009;6:24.

    07. Polichetti G, et al. Effects of particulate matter (PM10, PM2.5 and PM1) on the cardiovascular system. Toxicol 2009;261:1–8.

    08. Environmental Protection Agency. Particle pollution and your health. Available at: Last accessed: 22 July 2018.

    09. Interprofessional Technical Centre for Studies on Air Pollution (CITEPA). Particulate matter. Available at: Last accessed: 22 July 2018.

    10. Achilleos S, et al. Acute effects of fine particulate matter constituents on mortality: a systematic review and meta-regression analysis. Environ Int 2017;109:89–100.

    11. World Health Organization. Ambient (outdoor) air quality and health. Available at: Last accessed: 18 July 2018.

    12. Salvi S, Holgate ST. Mechanisms of particulate matter toxicity. Clin Exp Allergy 1999;29:1187–94.

    13. Traboulsi H, et al. Inhaled pollutants: the molecular scene behind respiratory and systemic diseases associated with ultrafine particulate matter. Int J Mol Sci 2017;18:243.

    14. Shusterman D. The effects of air pollutants and irritants on the upper airway. Proc Am Thorac Soc 2011;8:101–5.

    15. Lu CY, et al. Building-related symptoms among office employees associated with indoor carbon dioxide and total volatile organic compounds. Int J Environ Res Public Health 2015;12:5833–45.

    16. Cooper SC. The Truth about Mold. Dearborn Financial Publishing: Chicago, IL; 2004.

    17. Alessandrini ER, et al. Association Between Short-Term Exposure to PM2.5 and PM10 and Mortality in Susceptible Subgroups: A Multisite Case-Crossover Analysis of Individual Effect Modifiers. Am J Epidemiol 2016;184(10):744–54.

    18. Anderson JO, et al. Clearing the air: a review of the effects of particulate matter air pollution on human health. J Med Toxicol 2012;8(2):166–75.

    19. World Health Organization. Frequently Asked Questions. Available at: Last accessed: 23 July 2018.

    20. World Health Organization. Health effects of particulate matter. Available at: Last accessed: 25 July 2018.

    21. Xing YF, et al. The impact of PM2.5 on the human respiratory system. J Thorac Dis 2016;8(1):E69–74.

    22. Munkholm M, Mortensen J. Mucociliary clearance: pathophysiological aspects. Clin Physiol Funct Imaging 2014;34:171–7.

    23. Macy E. A rhinitis primer for family medicine. Perm J 2012;16:61–6.

    24. Balmes JR, et al. Symptomatic bronchoconstriction after short-term inhalation of sulfur dioxide. Am Rev Respir Dis 1987;136:1117–21.

    25. Kagawa J. Evaluation of biological significance of nitrogen oxides exposure. Tokai J Exp Clin Med 1985;10:348–53.

    26. Gehring U, et al. Traffic-related air pollution and respiratory health during the first 2 yrs of life. Eur Respir J 2002;19:690–8.

    27. Morgenstern V, et al. Respiratory health and individual estimated exposure to traffic-related air pollutants in a cohort of young children. Occup Environ Med 2007;64:8–16.

    28. Uysal N, Schapira, RM. Effects of ozone on lung function and lung diseases. Curr Opin Pulm Med 2003;9:144–50.

    29. Lin YK, et al. Temperature, nitrogen dioxide, circulating respiratory viruses and acute upper respiratory infections among children in Taipei, Taiwan: a population-based study. Environ Res 2013;120:109–18.

    30. Tam WW, et al. Association between air pollution and general outpatient clinic consultations for upper respiratory tract infections in Hong Kong. PLoS One 2014;9:e86913.

    31. Lappalainen S, et al. Indoor air particles in office buildings with suspected indoor air problems in the Helsinki area. Int J Occup Med Environ Health 2013;26:155–64.

    32. Fleischer NL, et al. Outdoor air pollution, preterm birth, and low birth weight: analysis of the World Health Organization Global Survey on Maternal and Perinatal Health. Environ Health Perspect 2014;122:425–30.

    33. Pedersen M, et al. Ambient air pollution and low birthweight: a European cohort study (ESCAPE). Lancet Resp Med 2013;1:695–704.

    34. Shah PS, et al. Air pollution and birth outcomes: a systematic review. Environ Int 2011;37:498–516.

    35. Deng Q, et al. Exposure to outdoor air pollution during trimesters of pregnancy and childhood asthma, allergic rhinitis, and eczema. Environ Res 2016;150:119–27.

    36. Hehua Z, et al. The impact of prenatal exposure to air pollution on childhood wheezing and asthma: a systematic review. Environ Res 2017;159:519–30.

    37. Esposito S, et al. Impact of air pollution on respiratory diseases in children with recurrent wheezing or asthma. BMC Pulm Med 2014,14:130.

    38. Gauderman WJ, et al. The effect of air pollution on lung development from 10 to 18 years of age. N Engl J Med 2004;351:1057–67.

    39. Gehring U, et al. Air pollution exposure and lung function in children: the ESCAPE project. Environ Health Perspect 2013;121:1357–64.

    40. Sacks JD, et al. Particulate matter-induced health effects: who is susceptible? Environ Health Perspect 2011;119:446–54.

    41. Simoni M, et al. Adverse effects of outdoor pollution in the elderly. J Thorac Dis 2015;7:34–45.

    42. Jiang XQ, et al. Air pollution and chronic airway diseases: what should people know and do? J Thorac Dis 2016;8(1):E31–40.

    43. Korten I, et al. Air pollution during pregnancy and lung development in the child. Paediatr Respir Rev 2017;21:38–46.

    44. van den Hooven EH, et al. Air pollution exposure during pregnancy, ultrasound measures of fetal growth, and adverse birth outcomes: a prospective cohort study. Environ Health Perspect 2012;120:150–6.

    45. Environmental Protection Agency. Air Pollution and Pregnancy. Available at: Last accessed: 22 July 2018.

    46. Gilliland FD. Outdoor air pollution, genetic susceptibility, and asthma management: opportunities for intervention to reduce the burden of asthma. Pediatrics 2009;123 Suppl 3:S168–73.

    47. Holloway JW, et al. Genomics and the respiratory effects of air pollution exposure. Respirology 2012;17:590–600.

    48. Ji H, Khurana Hershey GK. Genetic and epigenetic influence on the response to environmental particulate matter. J Allergy Clin Immunol 2012;129:33–41.

    49. Strohl KP, et al. Mechanical properties of the upper airway. Compr Physiol 2012;2:1853–72.

    50. de Grove KC, et al. Insights in particulate matter-induced allergic airway inflammation: focus on the epithelium. Clin Exp Allergy 2018;48:773–86.

    51. Wanner A, et al. Mucociliary clearance in the airways. Am J Respir Crit Care Med 1996;154:1868–902.

    52. Knowles MR, Boucher RC. Mucus clearance as a primary innate defense mechanism for mammalian airways. J Clin Invest 2002;109:571–7.

    53. Beule AG. Physiology and pathophysiology of respiratory mucosa of the nose and the paranasal sinuses. GMS Curr Top Otorhinolaryngol Head Neck Surg 2010;9:Doc07.

    54. Xia T, et al. Pulmonary diseases induced by ambient ultrafine and engineered nanoparticles in twenty-first century. Natl Sci Rev 2016;3:416–29.

    55. Liu YY, Di YP. Effects of second hand smoke on airway secretion and mucociliary clearance. Front Physiol 2012;3:342.

    56. Holgate ST, et al. Air Pollution and Health. San Diego, CA: Academic Press; 2009.

    57. Hiraiwa K, van Eeden SF. Contribution of lung macrophages to the inflammatory responses induced by exposure to air pollutants. Mediators Inflamm 2013;2013:619523.

    58. Fujii T, et al. Interaction of alveolar macrophages and airway epithelial cells following exposure to particulate matter produces mediators that stimulate the bone marrow. Am J Respir Cell Mol Biol 2002;27:34–41.

    59. Liu Y, et al. Impact of hydrogel nanoparticle size and functionalization on in vivo behavior for lung imaging and therapeutics. Mol Pharm 2009;6:1891–902.

    60. Chen BY, et al. The association of ambient air pollution with airway inflammation in schoolchildren. Am J Epidemiol 2012;175:764–74.

    61. Guan WJ, et al. Impact of air pollution on the burden of chronic respiratory diseases in China: time for urgent action. Lancet 2016;388:1939–51.

    62. Yoshida T, et al. Urban aerosols induce pro-inflammatory cytokine production in macrophages and cause airway inflammation in vivo. Biol Pharm Bull 2010;33:780–3.

    63. Gomes EC, et al. Impact of heat and pollution on oxidative stress and CC16 secretion after 8 km run. Eur J Appl Physiol 2011;111:2089–97.

    64. Filippidou EC, Koukouliata A. Ozone effects on the respiratory system. Prog Health Sci 2011;1:144–55.

    65. Romieu I, et al. Air pollution, oxidative stress and dietary supplementation: a review. Eur Respir J 2008;31:179–97.

    66. Brandt EB, et al. Exposure to allergen and diesel exhaust particles potentiates secondary allergen-specific memory responses, promoting asthma susceptibility. J Allergy Clin Immunol 2015;136:295–303.e297.

    67. Khreis H, et al. Exposure to traffic-related air pollution and risk of development of childhood asthma: a systematic review and meta-analysis. Environ Int 2017;100:1–31.

    68. Bartra J, et al. Air pollution and allergens. J Investig Allergol Clin Immunol 2007;17 Suppl 2:3–8.

    69. Mookherjee N, et al. Inhaled diesel exhaust alters the allergen-induced bronchial secretome in humans. Eur Respir J 2018;51:pii:1701385.

    70. Chauhan AJ, et al. Exposure to nitrogen dioxide (NO2) and respiratory disease risk. Rev Environ Health 1998;13:73–90.



man with mask


When avoiding air pollution is difficult, introducing measures to prevent the effects of air pollution and minimise respiratory symptoms can help.


female doctor


Almost every country in the world is affected by air pollution and exposure has been linked with a range of health effects, including increased morbidity and mortality, and respiratory diseases.