CHILD Cohort Study: Comprehensive Environmental Exposures

Between 2008 and 2012, the CHILD study enrolled 3624 pregnant mothers from the general population in four major cities across Canada (Vancouver, Edmonton, Winnipeg and Toronto) and a small rural population (Morden and Winkler) outside Winnipeg.³ Inclusion criteria were age >18 years (>19 years in Vancouver), living in proximity (<50 Km) to a participating delivery hospital, able to read, write and speak English, willing to donate cord blood, planning to deliver at a designated recruitment center participating hospital, and infant born at or after 35 weeks. Exclusion criteria were major congenital abnormalities or respiratory distress syndrome (RDS), expectation of moving away from a recruitment center within 1 year of recruitment, children of multiple births or resulting from in vitro fertilization, and children not spending over 80% of time in the index home.
Recruitment strategies ranged from having staff meet mothers in ante-natal ultrasound clinics and physician offices to community “baby fairs” and included person-to-person referrals and social media advertising. A representative general population was sought, as the development of allergy and asthma in a non-high-risk population may more readily identify environmental risks as well as novel genes. Over 80% of Canadians live in urban centers, and the recruited population should be generally representative of the Canadian population. Representativeness and potential biases (e.g., in socioeconomic status, allergic diathesis) will be determined by comparison with data from the total Canadian population.
Specific exposures of interest and their assessment methods were refined over a 2-year period, using input from an international planning workshop,^{11 (link)} consultation with housing experts representing the Canada Mortgage and Housing Corporation and experience gained from a preliminary study in a small Vancouver population. Finally, these refined methods were piloted with a Vanguard Cohort of 220 mothers and babies recruited at all four CHILD study sites¹² before implementation in the main cohort.¹³ This process led to an approach by which individual exposures were assessed across 15 domains (Table 1) using multiple methods or tools (Table 2), with emphasis on the home environment. Time-activity inside and outside the home is also recorded, focusing on rooms where the baby spent most time, awake and asleep, and time spent in transit and daycare. A representative detailed environmental questionnaire administered in early childhood is available in the Supplementary Information 1.
Several of the exposures listed in Tables 1 and 2 are linked to inflammatory responses of the acquired and innate immune systems.^{14 (link), 15 (link)} Many irritant and oxidizing exposures associated with asthma trigger the innate response including effects of second-hand tobacco smoke, cleaning chemicals, TRAP (e.g., nitrogen dioxide (NO₂)), mold and moisture, and chemicals that may be emitted inside the home.^{16 (link), 17 (link), 18 (link), 19 (link), 20 (link), 21} Also included in the CHILD study are exposures to the classically associated biologic allergens.^{22 (link)}