As a sweltering heat wave brought temperatures north of 90 degrees and high humidity to Massachusetts for the third straight day on Thursday, nearly 20 beaches across the state were closed, according to the state’s public water quality dashboard.

In all, 18 beaches across Massachusetts were closed as of the 9:30 a.m. update to the dashboard. The closures spanned from Great Barrington in Western Massachusetts to Boston in Eastern Massachusetts to beaches on Cape Cod and the Islands. Of the 18 beaches closed in the state as of Thursday morning, two-thirds — 12 of the 18 — were shuttered due to an excess of bacteria in the water.

The remaining closures were attributed to harmful cyanobacteria blooms and “other.”

• Read more: Here’s how hot it will get on Thursday in Massachusetts amid the heat wave

In Great Barrington, Lake Mansfield was closed, though the dashboard listed only “other” as a reason. In the eastern part of the state, in Boston’s Dorchester neighborhood, Tenean Beach was shuttered due to an excessive amount of bacteria in the water.

On the Cape, Attaquin Park in Mashpee was closed due to a harmful cyanobacteria bloom, the same reason given for the closure of Miacomet Pond in Nantucket.

Two beaches in Salem were closed on Thursday — the most in any municipality — with Camp Naumkeag closed due to “other” reasons and the back of Children’s Island closed because of bacteria.

• Read more: These beaches have the best — and worst — water quality in greater Boston, report finds

“If a beach is closed, do not swim or enter the water at that location to avoid risk of illness,” the dashboard warns.

The dashboard is updated twice a day, once in the morning and once in the afternoon, and shows the results of recent water quality tests at beaches across the state. More than 1,100 public and semi-public beaches in the state are regularly monitored.

And despite the closures, state officials say the beaches are still perfectly safe for recreational activities aside from swimming.

The full list of closures is below. If you can’t see the chart, click here.