While Cape Cod and much of the eastern United States sweltered under the blanket of a heat wave, Islanders and visitors, though still uncomfortably hot, suffered more from the high humidity this week. Temperatures peaked at 90 degrees, not unusually high for July according to National Weather Service meteorologist Kimberly Buttrick, but the high humidity, compounded by a lack of wind, is unusual. And it has made spending time outside uncomfortable.

“What makes the heat oppressive is that the dew points are high,” Ms. Buttrick said. “When you get dew points in upper 60s and low 70s, that really makes it feel uncomfortable, no matter what the temperature.”

The heat poses a threat to the health of the elderly and the very young, as well as domesticated animals. Many heat-stroke victims are seniors who can be incapable of moving to a cooler environment if their home does not have air conditioning. “Basically we are asking people to slow down, avoid strenuous activity particularly during the hottest part of the day, and to continually hydrate using, preferably, water,” said Peter Judge, public information officer at the Massachusetts Emergency Management Agency. “Be very conscious of elderly people, young kids and pets, as the heat can affect them.”

The Island’s senior centers became a refuge from the heat for many individuals.

In Oak Bluffs seniors turned up at the center in that town to bask in the air conditioning, watch television, play cards and exercise. In Edgartown, there is a waiting list to attend noontime meals, held on Tuesdays and Fridays. “When it got hot, people really started taking advantage” of the facility, said Peter Mohair, administrator there for two and a half years.

As residents cranked up their air conditioners to cool off along the Cape, the power company NStar struggled to meet demand and a request went out for voluntary conservation measures. ISO New England, the regional transmission organization, asked consumers to conserve electricity whenever possible.

“Basically we are recommending people put off activities that they can, so we can minimize the use of electricity,” during peak hours between noon and 8 p.m. according to Mr. Judge at MEMA. “People have got to be economical with their power over the next couple of days. We don’t want people to put their health in jeopardy, but for most people it’s just an inconvenience.”

Cash & Carry in Vineyard Haven just barely kept up with demand for ice as businesses, residents and wedding caterers filed orders all week. “As fast as we can bring it in, it sells,” said Jonathan Averill, the store owner. They also sold a lot of Gatorade and bottled water.

It seemed those who had previously postponed the purchase of cooling appliances found they could no longer procrastinate. Edgartown Hardware sold 150 fans and 60 air conditioners in the past seven days. These sales are way up from last year, said Linda Metell, assistant manager. “We can’t keep them on the shelves,” she said. The store has placed orders for more cooling products. “People are just trying to get some relief somehow,” she said. The store is also selling out fast of beach equipment, including inflatables and umbrellas, as well as kiddie pools, which some say are for their dogs.

“What we get concerned about with the heat is hyperthermia,” said certified veterinary technician Terry Lowe, who manages the Animal Health Care Associates practice at the airport. At her practice, she’s treated dogs left in cars or left outside who suffer from heat stroke. Dogs sweat through their paws and their tongues, which amounts to a small surface area compared to the human skin. She said dogs should not be left in cars even with the windows cracked in this heat. “That metal car is still going to act like an oven,” she said. Dogs left outside can also be the victims of flies, who may lay eggs in their fur and grow into maggots. They should also not be brought along for a jog or a run in high heat, she said.

Cats may be a bit smarter, she said, so they don’t tend to pose the same risks. They are better at seeking shady spots, and don’t tend to be brought along on car rides.

Temperatures are expected to cool down slightly this weekend with the arrival of thunderstorms.