Overview of the identified risk factors for COPD and their corresponding effect sizes
| Category | Risk factor | Number of reviews | Effect measure | Effect size (95% CI) |
| Air pollution and chemical exposures | NO2 exposure | 2 | HR/risk ratio | HR: 1.07 (1.00–1.16) [60]; risk ratio: 1.02 [82] |
| Ambient PM2.5 exposure | 2 | HR/OR | HR: 1.18 (1.13–1.23) [60]; OR: 1.73 (1.16–2.58) [72] | |
| Indoor biomass burning | 15 | OR | Range from 1.52 (1.39–1.67) [61] to 3.16 (1.82–5.49) [69] | |
| Pesticide exposure | 1 | OR | Range from 1.05 (0.74–1.51) to 4.10 (2.20–6.30) [46] | |
| Living in polluted areas | 1 | OR | 1.63 (1.20–2.21) [57] | |
| VGDF exposure | 1 | OR | 1.43 (1.19–1.73) [64] | |
| Early life exposures | Low birth weight | 1 | OR | 1.58 (1.08–2.32) [50] |
| Childhood wheeze | 1 | Risk ratio | 5.31 (1.03–27.27) [68] | |
| Early life respiratory problems | 6 | OR | Range from 2.23 (1.63–3.07) [50] to 3.44 (1.33–8.90) [72] | |
| Childhood asthma | 4 | OR | Range from 3.00 (2.25–4.0) [80] to 7.87 (5.40–11.45) [52] | |
| Child maltreatment | 1 | OR | 1.30 (1.18–1.42) [50] | |
| Childhood environmental tobacco smoke exposure | 2 | OR/risk ratio | OR: 1.30 (0.83–1.61) [50]; risk ratio: 0.88 (0.72–1.07) [92] | |
| Maternal smoking | 1 | OR | 1.42 (1.17–1.72) [50] | |
| Pre-term birth | 1 | OR | 1.17 (0.087–1.58) [50] | |
| Nutrition and weight status | BMI between 25.0 and 29.9 kg·m−2 | 3 | OR | Range from 0.80 (0.73–0.87) [74] to 0.96 (0.76–1.22) [72] |
| BMI ≥30 kg·m−2 | 1 | OR | 0.86 (0.73–1.02) [74] | |
| Low BMI | 5 | OR | Range from 3.83 (2.22–6.60) [57] to 1.96 (1.78–2.17) [74] | |
| Consumption of red meat | 2 | HR | 1.08 (1.03–1.13) [78] | |
| Western style/unhealthy diet | 3 | OR | 2.12 (1.64–2.74) [63], 1.22 (0.84–1.76) [51] | |
| Vitamin D deficiency | 1 | OR | 1.77 (1.18–2.64) [79] | |
| Drinking history | 1 | OR | 0.82 (0.54–1.23) [72] | |
| Healthy diet | 1 | OR | 0.88 (0.82–0.94) [51] | |
| Occupational exposures | Dust exposure (wood, mineral) | 11 | OR | Range from 0.97 (0.68–1.39) [79] to 1.79 (1.15–2.79) [57] |
| Workplace irritants | 1 | NA | NA [44] | |
| Agricultural work | 2 | OR | 1.77 (1.50–2.08) [67] | |
| Physiological, personal, and socioeconomic characteristics | Smoking | 12 | OR | Range from 2.89 (2.63–3.17) [90] to 5.50 (4.20–7.20) [70] |
| Second-hand smoke | 6 | Risk ratio/OR | Range from 1.20 (1.08–1.34) [92] to 1.56 (1.40–1.74) [79] | |
| Chewing tobacco | 1 | OR | 12.90 (3.40–49.40) [70] | |
| Parental COPD | 4 | OR | 1.57 (1.29–1.93) [65], 2.07 (1.47–2.92) [57] | |
| Recurrent respiratory infections | 1 | OR | 15.02 (4.54–49.68) [57] | |
| History of rheumatoid arthritis | 1 | OR | 1.99 (1.61–2.45) [79] | |
| History of psoriasis | 1 | OR | 1.45 (1.21–1.73) [79] | |
| History of tuberculosis | 4 | OR | 2.80 (1.90–4.00) [85], 5.98 (4.18–8.56) [61] | |
| Waterpipe smoking | 1 | OR | 3.18 (1.25–8.09) [79] | |
| Poor housing ventilation | 1 | OR | 3.99 (1.24–12.82) [57] | |
| Low SES | 5 | OR | Range from 0.80 (0.50–1.30) [62] to 1.61 (1.21–2.15) [57] | |
| Age | 4 | OR | Range from 1.50 (1.30–1.50) [85] to 4.70 (3.50–6.40) [70] | |
| Male sex | 3 | OR | Range from 1.47 (1.10–1.96) [57] to 2.10 (1.90–2.30) [85] | |
| Serum homocysteine | 1 | MD | 3.05 [91] | |
| Inflammatory markers in serum | 1 | SMD | Range from 0.87 (0.44–1.31) [53] to 2.34 (0.69–4.00) [53] | |
| Traffic intensity | 1 | OR | 1.26 (0.95–1.70) [79], 1.30 (0.92–1.82) [79] | |
| Living at high altitudes | 1 | OR | 1.18 (0.85–1.62) [59] | |
| Living in urban areas | 1 | OR | 1.20 (1.00–1.50) [85] | |
| Living in rural areas | 1 | OR | 1.40 (1.30–1.60) [85] |
95% confidence intervals are given when available. BMI: body mass index; HR: hazard ratio; PM: particulate matter; (S)MD: (standardised) mean difference; NA: not assigned, indicates studies without a meta-analysis and without effect sizes; SES: socioeconomic status; VGDF: vapours, gases, dusts and fumes.