If you look further down in the paper, they mention the variables they adjusted for:

> [..] all models were adjusted for sex, birth year, area-level urbanicity, area-level morbidity, area-level educational level, area-level employ- ment rate, school class size, school’s ninth grade size, parental educational level, parental income, and parental mental health.

So a contaminant or other environmental factor that didn't correlate perfectly with those factors wouldn't be corrected for. How could it be, without knowing what it is?