By Melissa M. Werthmann Globe Correspondent

State officials have issued an air quality alert today for a large swath of the state because of increased pollutant levels and ozone readings that could exceed health standards.

The Department of Environmental Protection rates air quality on a scale from zero to 300. Values today for some areas in western, central, and northeastern Massachusetts are estimated to rise to 100 to 120. Anything over 100 is classified as “unhealthy for sensitive groups.” People with heart disease or lung disease, such as asthma, are advised to reduce heavy activity outside, the DEP said.

The alerts were issued for areas in Essex, Hampden, Hampshire, Middlesex, and Worcester counties.

“Our airmass that we’ve been under the influence of for the past several days has gotten a little dirtier and a little more humid and as a result we’re expecting some high levels of ozone to develop today,” the DEP’s air quality hotline said.

Air quality index values for other parts of the state are expected to be 50 to 70 today, which is considered “moderate” by the DEP. Values for Cape Cod are expected to be in the 40s, considered “good” by the DEP.

On Saturday, the plume of bad air will shift slightly to the south, with the highest readings, 80 to 105, expected in southeastern Massachusetts, as well as the Cape and islands, according to the hotline. On Sunday, air quality is expected to improve across the area.

Aside from an air quality advisory, today will bring mostly sunny skies and temperatures in the low 90s.

The high temperatures are no surprise, as Massachusetts residents have felt the heat lately, but National Weather Service forecaster Charlie Foley said the humidity will start climbing today, becoming oppressive over the weekend.