1. Biomarkers Definitions Working Group. Biomarkers and surrogate endpoints: preferred definitions and conceptual framework. Clin Pharmacol Ther 2001;69:89–95.

2. Califf RM. Biomarker definitions and their applications. Exp Biol Med (Maywood) 2018;243:213–221.

3. Kelly FJ, Fussell JC. Air pollution and public health: emerging hazards and improved understanding of risk. Environ Geochem Health 2015;37:631–649.

4. World Health Organization. Regional Office for Europe. Health effects of particulate matter: policy implications for countries in eastern Europe, Caucasus and central Asia Copenhagen: World Health Organization, c2013. [cited 2023 Sep 1]. Available from: https://iris.who.int/handle/10665/344854 . 5. Kim KH, Kabir E, Kabir S. A review on the human health impact of airborne particulate matter. Environ Int 2015;74:136–143.

6. Peters A, Wichmann HE, Tuch T, Heinrich J, Heyder J. Respiratory effects are associated with the number of ultrafine particles. Am J Respir Crit Care Med 1997;155:1376–1383.

7. Kim D, Chen Z, Zhou LF, Huang SX. Air pollutants and early origins of respiratory diseases. Chronic Dis Transl Med 2018;4:75–94.

8. Zhao J, Li M, Wang Z, et al. Role of PM2.5 in the development and progression of COPD and its mechanisms. Respir Res 2019;20:120.

9. Yang X, Zhang T, Zhang Y, Chen H, Sang S. Global burden of COPD attributable to ambient PM2.5 in 204 countries and territories, 1990 to 2019: a systematic analysis for the Global Burden of Disease Study 2019. Sci Total Environ 2021;796:148819.

10. Han C, Oh J, Lim YH, Kim S, Hong YC. Long-term exposure to fine particulate matter and development of chronic obstructive pulmonary disease in the elderly. Environ Int 2020;143:105895.

11. Bloemsma LD, Hoek G, Smit LAM. Panel studies of air pollution in patients with COPD: systematic review and meta-analysis. Environ Res 2016;151:458–468.

12. DeVries R, Kriebel D, Sama S. Outdoor air pollution and COPD-related emergency department visits, hospital admissions, and mortality: a meta-analysis. COPD 2017;14:113–121.

13. Dominici F, Peng RD, Bell ML, et al. Fine particulate air pollution and hospital admission for cardiovascular and respiratory diseases. JAMA 2006;295:1127–1134.

14. Gan WQ, FitzGerald JM, Carlsten C, Sadatsafavi M, Brauer M. Associations of ambient air pollution with chronic obstructive pulmonary disease hospitalization and mortality. Am J Respir Crit Care Med 2013;187:721–727.

15. Lee KY, Chiang LL, Ho SC, et al. Associations of autophagy with lung diffusion capacity and oxygen saturation in severe COPD: effects of particulate air pollution. Int J Chron Obstruct Pulmon Dis 2016;11:1569–1578.

16. Audi C, Baïz N, Maesano CN, et al. Serum cytokine levels related to exposure to volatile organic compounds and PM2.5 in dwellings and workplaces in French farmers - a mechanism to explain nonsmoking COPD. Int J Chron Obstruct Pulmon Dis 2017;12:1363–1374.

17. Gao N, Xu W, Ji J, et al. Lung function and systemic inflammation associated with short-term air pollution exposure in chronic obstructive pulmonary disease patients in Beijing, China. Environ Health 2020;19:12.

18. Garshick E, Grady ST, Hart JE, et al. Indoor black carbon and biomarkers of systemic inflammation and endothelial activation in COPD patients. Environ Res 2018;165:358–364.

19. Huang S, Garshick E, Vieira CLZ, et al. Short-term exposures to particulate matter gamma radiation activities and biomarkers of systemic inflammation and endothelial activation in COPD patients. Environ Res 2020;180:108841.

20. Busenkell E, Collins CM, Moy ML, et al. Modification of associations between indoor particulate matter and systemic inflammation in individuals with COPD. Environ Res 2022;209:112802.

21. Chen R, Qiao L, Li H, et al. Fine particulate matter constituents, nitric oxide synthase DNA methylation and exhaled nitric oxide. Environ Sci Technol 2015;49:11859–11865.

22. Grady ST, Koutrakis P, Hart JE, et al. Indoor black carbon of outdoor origin and oxidative stress biomarkers in patients with chronic obstructive pulmonary disease. Environ Int 2018;115:188–195.

23. Huang S, Koutrakis P, Grady ST, et al. Effects of particulate matter gamma radiation on oxidative stress biomarkers in COPD patients. J Expo Sci Environ Epidemiol 2021;31:727–735.

24. Huang Q, Hu D, Wang X, et al. The modification of indoor PM2.5 exposure to chronic obstructive pulmonary disease in Chinese elderly people: a meet-in-metabolite analysis. Environ Int 2018;121(Pt 2):1243–1252.

25. Belli AJ, Bose S, Aggarwal N, et al. Indoor particulate matter exposure is associated with increased black carbon content in airway macrophages of former smokers with COPD. Environ Res 2016;150:398–402.

26. Abramson MJ, Wigmann C, Altug H, Schikowski T. Ambient air pollution is associated with airway inflammation in older women: a nested cross-sectional analysis. BMJ Open Respir Res 2020;7:e000549.

27. Manney S, Meddings CM, Harrison RM, et al. Association between exhaled breath condensate nitrate + nitrite levels with ambient coarse particle exposure in subjects with airways disease. Occup Environ Med 2012;69:663–669.

28. Rama TA, Paciência I, Cavaleiro Rufo J, et al. Exhaled breath condensate pH determinants in school-aged children: a population-based study. Pediatr Allergy Immunol 2021;32:1474–1481.

29. Klümper C, Krämer U, Lehmann I, et al.; GINIplus and LISAplus study groups. Air pollution and cytokine responsiveness in asthmatic and non-asthmatic children. Environ Res 2015;138:381–390.

30. Zahedi A, Hassanvand MS, Jaafarzadeh N, Ghadiri A, Shamsipour M, Dehcheshmeh MG. Effect of ambient air PM2.5-bound heavy metals on blood metal(loid)s and children’s asthma and allergy pro-inflammatory (IgE, IL-4 and IL-13) biomarkers. J Trace Elem Med Biol 2021;68:126826.

31. Gao J, Xu X, Ying Z, et al. Post-effect of air quality improvement on biomarkers for systemic inflammation and microparticles in asthma patients after the 2008 Beijing olympic games: a pilot study. Inflammation 2017;40:1214–1224.

32. Strand M, Rabinovitch N. Health effects of concurrent ambient and tobacco smoke-derived particle exposures at low concentrations in children with asthma. J Expo Sci Environ Epidemiol 2020;30:785–794.

33. He L, Norris C, Cui X, et al. Role of endogenous melatonin in pathophysiologic and oxidative stress responses to personal air pollutant exposures in asthmatic children. Sci Total Environ 2021;773:145709.

34. Mohd Isa KN, Hashim Z, Jalaludin J, Lung Than LT, Hashim JH. The effects of indoor pollutants exposure on allergy and lung inflammation: an activation state of neutrophils and eosinophils in sputum. Int J Environ Res Public Health 2020;17:5413.

35. Kim S, Seo H, Lee Y, Park J. Study design algorithm for medical literature of intervention (DAMI) and risk of bias for non-randomized studies (robans) ver 2.0 by HIRA. Seoul: Health Insurance Review & Assessment Service, 2013;65–66. 36. Fireman Klein E, Adir Y, Krencel A, et al. Ultrafine particles in airways: a novel marker of COPD exacerbation risk and inflammatory status. Int J Chron Obstruct Pulmon Dis 2019;14:557–564.

37. Fireman Klein E, Adir Y, Fireman E, Kessel A. Cigarette-related cadmium and environmental pollution exposure are reflected in airway ultrafine particle content. ERJ Open Res 2020;6:00361–2019.

38. Tang L, Shi S, Wang B, et al. Effect of urban air pollution on CRP and coagulation: a study on inpatients with acute exacerbation of chronic obstructive pulmonary disease. BMC Pulm Med 2021;21:296.

39. He L, Norris C, Cui X, et al. Oral cavity response to air pollutant exposure and association with pulmonary inflammation and symptoms in asthmatic children. Environ Res 2022;206:112275.

40. Tejwani V, Woo H, Liu C, et al. Black carbon content in airway macrophages is associated with increased severe exacerbations and worse COPD morbidity in SPIROMICS. Respir Res 2022;23:310.

41. Sampson PD, Szpiro AA, Sheppard L, Lindström J, Kaufman JD. Pragmatic estimation of a spatio-temporal air quality model with irregular monitoring data. Atmos Environ 2011;45:6593–6606.

42. Szpiro AA, Sampson PD, Sheppard L, Lumley T, Adar SD, Kaufman J. Predicting intra-urban variation in air pollution concentrations with complex spatio-temporal dependencies. Environmetrics 2009;21:606–631.

43. Zusman M, Gassett AJ, Kirwa K, et al. Modeling residential indoor concentrations of PM2.5, NO2, NOx, and secondhand smoke in the Subpopulations and Intermediate Outcome Measures in COPD (SPIROMICS) Air study. Indoor Air 2021;31:702–716.

44. He L, Li Z, Teng Y, et al. Associations of personal exposure to air pollutants with airway mechanics in children with asthma. Environ Int 2020;138:105647.

45. Lam PK, Gray JS. The use of biomarkers in environmental monitoring programmes. Mar Pollut Bull 2003;46:182–186.

46. Kim J, Kim NY, Kim WJ. Biomarkers of particulate matter exposure in patients with chronic obstructive pulmonary disease: a systematic review. J Thorac Dis 2023;15:3453–3465.