Exposure

Exposure information was collected through a novel, self-administered questionnaire (EnvIMS-Q) that had been tested for reliability, cross-cultural validity and perceived difficulty of completion.¹⁶ In Norway, the level of education was reported on a five-point scale including “7 years or less” (elementary school), “8–10 years” (middle school), “11–13 years” (high school), “14 years or more” (college/university) and “I do not know.” Covariates reflecting risk factors most commonly associated with MS risk included history of smoking, history of infectious mononucleosis, proxies of vitamin D levels (outdoor activity and sun exposure, vitamin D supplementation, fatty fish intake) and body size.

Smoking habits were reported as “ever” and “never” smoker, cigarettes/day (1–4, 5–10, 11–20 and 20+) in specific age periods (11–15, 16–20, 21–25 and 26–30), age at smoking initiation and total years of smoking. Those who started smoking after disease onset were classified as “never” smokers. History of infectious mononucleosis was reported as “yes,” “no” and “I do not remember.” Frequency of outdoor activity, a proxy for sun exposure and vitamin D levels, was reported for both winter and summer in the specific age periods 0–6, 7–12, 13–15, 16–18, 19–24 and 25–30. It was reported as “virtually all the time,” “quite often,” “reasonably often” and “not that often.” The frequency of use of vitamin D-containing supplements (e.g. cod liver oil) in the age period 13–19 was reported as “never/seldom,” “1–3 times/month,” “1 time/week,” “2–3 times/week,” “4–6 times/week” and “7+ times/week.” Information on intake of specific fish species (herring, mackerel, halibut, flounder, salmon and trout) was reported as “never/seldom,” “1 time/month,” “2–3 times/month,” “1 time/week,” “2 times/week” and “3 and more times/week.” A figure rating scale consisting of body sketches, which reflect individuals’ body mass index (BMI),²⁰ was used as an estimate for BMI. This scale was used for the specific ages 5, 10, 15, 20, 25 and 30.