Aamjiwnaang First Nation reported "high levels" of sulphur dioxide through its notification system Wednesday.
According to the Clean Air Sarnia and Area website, the Ada Lockridge Rogers monitoring station measured 128 parts per billion of sulphur dioxide at mid-afternoon., in the "poor" range.
Conditions were improving gradually, according to the site.
Residents living north of the CN tracks on Tashmoo Avenue were being advised to close windows, turn off air conditioners, and remain inside.
Buildings at the First Nation were closed Wednesday morning due to high levels of benzene detected at the Sportscenter Monitoring Station.
