Commentary Table.

Characteristics of Four Danish Study Populations Used to Investigate Associations Between Exposure to Ambient Air Pollution and Childhood Asthma

	Study Populations
	Nationwide Cohort	DNBC	COPSAC2010	COPSAC2000 + COPSAC2010
Study area	Denmark (nationwide)	Denmark (nationwide)	Zealand, Denmark	Zealand, Denmark, and Greater Copenhagen, Denmark
Years of birth	1998–2016	1998–2003	2008–2010	1998–2001, 2008–2010
Population size	1,060,154	22,084	700	803
Inclusion/ Exclusion criteria	Live-born singletons with data on exposures and maternal data on education, income, smoking, asthma, and parity	Live-born singletons with data on exposures, DNBC interview 1, asthma at 7 years, asthma at 11 years, and maternal data on education, income, smoking, asthma, parity, and breastfeeding	Live-born with data on exposures, outcomes, and covariates	Live-born singletons with data on exposure, outcomes, and covariates
Outcomes	Asthma Incidence (ICD-10 codes)	Asthma incidence (ICD-10 codes), asthma prevalence at age 7 (parental recall of physician- diagnosed asthma)	Cytokines in blood from 6 months of age Nasal epithelial DNA methylation and gene expression at age 4 weeks Asthma prevalence at age 6 (parental recall and clinical judgment) Allergic sensitization prevalence at age 6 (skin prick test) Allergic rhinitis at age 6 (parental recall and clinical judgment)	Lung function at age 6 (forced expiratory volume in 1 second, FEV1)
Statistical analyses	Incidence: Cox proportional hazards models	Incidence: Cox proportional hazards models Prevalence: logistic regression and Poisson regression	Linear regression	Mixed-effect linear regression
Single- and two- pollutant models	Single- and two- pollutant models	Single-pollutant models	Single-pollutant models	Single- and two-pollutant models

COPSAC = Copenhagen Prospective Studies on Asthma in Childhood, DNBC = Danish National Birth Cohort, ICD = International Classification of Diseases