TABLE 1

Summary of COPD risk factors operationalised with respect to life period

COPD risk factor		Critical life period and age of cohort members when data were collected							Cumulative exposure
Category	Individual factor	In utero/at birth	Childhood			Adulthood
Category	Individual factor	0 years of age	7 years of age	11 years of age	16 years of age	23 years of age	33 years of age	42 years of age
Smoking exposure	Passive smoke exposure	Smoking status during pregnancy (4 months)			Living with a smoker	Living with smoker at any stage			Combined passive smoking exposure (in utero, childhood and adulthood) personal smoking in adulthood
Smoking exposure	Personal smoking status					Persistent smoker ever smoker, and never smoker
SC	SC (occupation and classified as: professional/managerial, unskilled nonmanual, skilled manual, unskilled manual)	SC of male head		SC of male head				Own SC	Combined social class at birth, 11 and 42 years (range 0–9), with higher scores indicating lower social class
Asthma	Asthma		Report of ever having asthma or bronchitis				Ever asthma in adulthood		Childhood and adulthood reporting of asthma
Others	Low birth weight	Birth weight <2.5 kg
	Preterm birth	Gestation age <37 weeks
	Pneumonia		Ever had pneumonia
	Occupation exposure						Minimal or potential occupational exposure to dust, gases and fume
	Biomass exposure						Paraffin or solid fuel as heating source

SC: social class.