Eastern Hampden, Hampshire and Essex counties were the areas under a warning, which included cities like Gloucester and Springfield.
DEP Spokesman Ed Colletta said residents with respiratory issues like asthma, lung disease — and children — should be careful not to exert themselves outside in the areas affected.
Children's smaller lungs, and the fact that they breathe deeper when exercising, means they could take in more ozone.
Colletta said the wind could shift easily, so residents in other areas that aren't under a warning now should still take note.
WBZ's Chris Fama (@CFamaWBZ) reports: