Table 2. Dose-response relationship between amount of smoking and risk of developing CAP.

Trend (OR) of 1.xy means xy% increase of risk of CAP per increase in category documented in the ‘Quantification of smoking exposure’ column.

Study	Smoking status	Quantification of smoking exposure	Trend (OR)	p value	Association
Almirall 1999	Current & ex-smokers	Pack-years • 0 • 5–16.4 • 16.5–38 • >38	1.37, 95% CI 1.17–1.61	<0.001	Linear
Almirall 2008	Current & ex-smokers	Packs of cigarettes smoked daily x 365 x years smoked$ • 0 • 1–150 • 151–300 • >300	1.27, 95% CI 1.17–1.38	<0.001	Linear
Almirall 1999	Current smokers	Cigarettes smoked daily • 0 • 1–9 • 10–20 • >20	1.30, 95% CI 1.02–1.67	0.037	Non-linear
Farr 2000	Current smokers	Cigarettes smoked daily x years smoked • 0 • 1–225 • 226–578 • 579+	1.52, 95% CI 1.30–1.78	<0.001	Non-linear; effect mainly in the highest two categories of smoking (in bold)
Conley 1996	Current smokers	Packets of cigarettes daily • ≤ half (‘light’) • half to <2 (‘moderate’) • >2 (‘heavy’)	1.51, 95% CI 1.10–2.08	0.011	Non-linear

^$This formula was clarified directly by personal correspondence with study author.