The federal Environmental Protection Agency warned that unhealthy air quality is expected today in Rhode Island and in much of New England.

The EPA asked “the public to take action,” cutting air pollution by:

•Using public transportation, car pooling and/or combining trips.

•Refueling cars at night to reduce gasoline vapors getting into the air during the daytime when the sun can cook the vapors and form ozone.

•Avoiding the use of small gasoline-powered engines, such as lawn mowers, chain saws and leaf blowers.

The bad air may continue into the weekend. For information from the EPA about real-time air quality data and forecasts, go to epa.gov/ne/aqi.

Because of the alert, Rhode Island Public Transit Authority buses and trolleys, including express buses to beaches, will be free today.

A hot and humid air mass is visiting, which means unhealthy air quality levels, the state Department of Environmental Management says. Elevated ground level ozone concentrations are the cause of it — a big part of smog — and form through a reaction of pollutants coming from the exhaust of motor vehicles, industry and other sources when there are higher temperatures and sunlight.

The EPA said exposure to high ozone levels can cause serious breathing problems, aggravate asthma and other pre-existing lung diseases, and make people more prone to respiratory infection. People should stay away from strenuous outdoor activity, especially children and adults with respiratory problems.

The state Department of Health said unhealthy ozone can cause throat irritation, coughing, chest pain and shortness of breath — symptoms worsened by exercise and heavy activity. Children, elderly and people who have underlying lung diseases, such as asthma, are at particular risk.