Surviving Boston summer without air conditioning? Growing share of households say no.

It’s hard to fathom not having air conditioning, especially as New England is being baked by an early summer heat wave that is forecast to push Boston well above 90 degrees Thursday. For a growing share of Boston households, the cooling devices are becoming a must to endure excruciating heat.

In decades past, having an air-conditioning unit in Boston was considered a luxury good, according to Ryan Williams, president of 128 Plumbing Heating Cooling and Electric, which has served the Greater Boston area for 32 years. That’s changed since the 1970s, as local summertime temperatures in Boston have climbed by 1.5 degrees on average.

“Air conditioning is becoming a necessity,” Williams said. “You do not build a home today without air conditioning.”

Between 2015 to 2021, the most recent data available, the share of Boston households with primary air conditioning has increased by nearly 5 percentage points from 89.7 percent to 94.5 percent, according to data from the US Census Bureau’s American Housing Survey. That puts Boston closer in line with cities like notoriously sweltering Washington, D.C, where 98.5 percent of households have primary air-conditioning units.

The American Housing Survey is a major national survey that tracks various changes in the housing stock over time. Some of those statistics, like the share of households with air conditioning, are useful barometers to track how cities and states are shifting as climate change drives temperatures up.

It’s no surprise that muggy cities like Houston and Miami top the list, with 99.1 percent and 98.9 percent of households having air conditioning, respectively. On the other hand, cities like San Francisco, wheresummers are relatively cool, have a dramatically lower share of homes with primary air conditioning, at just 45.2 percent.

When temperatures spike, business is good for air-conditioning companies. Just this week, sky-high summer temperatures have prompted some local air-conditioning companies to draft up extra staff to man their phone lines in anticipation of a surge in calls. Leaders in the industry say it happens during almost every heat wave.

Globe meteorologist Ken Mahan says that it’s not just the headline-grabbing high daytime temperatures that have pushed Bostonians toward air conditioning. Another big motivating factor is that it’s just not getting as cool at night anymore.

During this heat wave, nighttime temperatures are expected to settle around 75 to 80 degrees, Mahan said. Without cooler temperatures at night during heat waves, experts say, people without air-conditioning systems are more likely to develop serious health conditions, like heat illness or heat stroke. That’s particularly true in heat waves that develop early in the summer before people have become acclimated to the higher temperatures.

And although higher summertime temperatures are clearly having a major impact on air-conditioning sales, there are other factors contributing to Boston’s embrace of air conditioning.

Technological advances in the air-conditioning industry have expanded the number of households that can easily set up an air-conditioning system. Williams says that newer systems like those that connect outdoor and indoor units without bulky air ducts have made it far easier to set up air-conditioning units in old homes that don’t have the design features that a traditional system required.

Also in the last several decades, new systems have become far more energy efficient, thanks to gradual improvements in conductors and other technologies, making them more appealing to both environmentally friendly and price-conscious consumers. Tax breaks and incentive programs to set up new energy efficient systems have also encouraged more consumers to buy new units.

And yet, even still, around 5.5 percent of Boston households still don’t have air conditioning.

For households without their own system set up, Mahan had some important advice: Keep the lights off as much as possible. Don’t use the oven. Make sure to keep windows open at night, even when nighttime temperatures are high, because even a slight breeze can still cool things down. Keep an eye on the National Weather Service’s HeatRisk app to see how bad things will get in your local area.

And if you do end up calling an air-conditioning company to install a new system, be patient. Their telephone lines probably won’t cool off until your temperature gauge does.