According to the alert, ground-level ozone will be climbing and is expected to reach unhealthy levels.
Officials say combined with the heat advisory in place, the effects of poor air quality will be even harsher.
Elevated ozone levels can cause reduced lung function and irritation in children, individuals with diseases like asthma or COPD and even healthy adults who exert themselves.
To protect your health, the DEP says to avoid strenuous activity during the afternoon.
Check out the Maine CDC website for info about impacts of extreme heat and appropriate actions to take.
Asthma information can be found by visiting the Maine CDC Asthma Prevention and Control Program at their web site. You can also visit the EPA's Web site to find information about asthma triggers and lessons on asthma management.
You can also visit the DEP's Air Quality Forecast page.