As is the case for many people, my home cannot be retrofitted with central air. My 100-plus-year-old Brooklyn apartment, which features prominently in my seven years of air quality reporting for WIRED, relies on window air conditioning units to keep cool on our warming planet. While there is the obvious paradox that air conditioners are players in climate change, AC units are evolving with more environmentally safe refrigerants, eco modes, smart apps, modern design, and energy-efficient consumption.

That is to say, while I prefer to keep my AC units turned off, on hot days, my top-floor apartment’s temperature will climb to the upper 90s without the support of air conditioning. The Centers for Disease Control and Prevention estimates that more than 700 people die every year in the United States due to extreme heat. The elderly, young children, pregnant women, and those with existing medical conditions are especially vulnerable.

The number one recommendation from the CDC during extreme heat events is to stay inside an air-conditioned location. The World Health Organization warns that while electric fans can move air onto the body to create a sort of wind-chill effect, when it’s 104 degrees Fahrenheit or above, they can actually increase your body temperature. The ideal combination is using air conditioning in concert with fans to move cool air around a space.

The inability to cool down one’s body temperature is what can lead to heat stroke and other heat-related health events. Air conditioning saves lives.

I’ve been writing about air quality for WIRED since 2018, and as climate change wreaks havoc on our environment, I continue to test and review the latest window air conditioning units on the market. There are two types of window units we review: the recognizable metal box units that hang out of a window, and the increasingly popular “portable” units that use hoses—preferably two—that attach to the window. Both types of AC units offer relief from the heat and protect against heat-related illness. If you’re unsure whether you should turn on your air conditioner, you can check the CDC’s HeatRisk Tracker Map.

Professional Installation Is Worth the Money

Your city or town may not have fines and requirements for window unit installation. New York City requires support brackets, especially for those units above sidewalks. I recently hired a professional AC company to securely install LG’s 12,000 BTU model, above. At over 80 pounds, it was too heavy for me to lift. I’ve yet to test a window unit AC where the manufacturer didn’t recommend two people for lifting and installing the unit, usually with a pictogram on the box of two people holding the same box.

Window units are designed to fit in single- or double-hung windows, usually at a minimum height opening of 13 inches. A window unit’s metal support brackets are designed to counteract the downward force of gravity. And there shouldn’t be any books, bricks, or blocks of wood used to level the unit once in the window. There are exceptions—window designs like U-Shaped ACs, like our pick below (an updated model from the one that was recalled this summer), distribute weight equally between the inside and outside of the window sash and may not require brackets.

Smart AC Units Save Money

More and more AC units connect to an app, and while some apps are easier to use than others, smart units have the added feature of turning them on and off remotely or through the use of in-app timers. Smart units have Eco Mode that run at a lower setting, as well as a variety of other features, including temperature settings. Smart ACs give the user so much more agency in knowing when to turn on or off their units.

Environmentally Friendly Refrigerant

In my hometown of New York City, residents need to make an online appointment with the Department of Sanitation when placing an old air conditioner on the sidewalk for trash collection. The city removes the CFC (chlorofluorocarbon) gas, or Freon, and affixes a sticker, clearing the unit to be picked for removal. There are now more environmentally friendly refrigerants on the market, such as R600a or R32. Windmill uses R32 in its line of ACs, like our pick above, and while refrigerants like R32 have fewer ozone-depleting properties than old Freon, it is flammable. It’s that flammability that keeps municipal waste management from collecting it for disposal. Companies like Windmill have a trade-in and recycling program for their customers. If your new AC has a yellow triangle with a black flame, your refrigerant is flammable.

Sound

The rattle from a window unit can vibrate throughout a window frame—another reason why professional installation is so important. If an AC isn’t properly leveled, there’s going to be noise. Beyond the rattle of an unleveled AC, would-be AC customers can log on to a manufacturer’s website or online store to see the decibel rating for each unit. Knowing that a refrigerator’s hum is around 40 to 50 decibels and that conversation is around 60 decibels can help you choose the right sound level for you. Knowing the decibel rating is especially important for bedroom units. Will your AC keep you up on hot nights?

Directional Airflow and Filters

I recently tested GE’s most affordable AC unit, above, and it was only when it was placed next to my bed that I noticed the limited choices I had in choosing the direction of airflow. It had a small grip to slide the airflow to the right, center, or left. They all felt very similar, and it was apparent that having the directional airflow option for straight-up or close to straight-up airflow is a must-have. There is little choice in where to place a window AC in a one-window room. Look for louvers that can move both up and down as well as side to side. WIRED has tested ACs with HEPA filters and with regular slide-out filters. It’s important to keep your AC’s filters clean for your unit to run properly. A clean-the-filter indicator light is an especially helpful feature.

Portable ACs for Unusual Windows

Portable air conditioners are often the only choice for those with either unique or sliding windows, or for those who can’t or aren’t allowed to install a unit in their window. It wasn’t that long ago that most units on the market had a single hose that fit into a window panel with a hole that matched the circumference of the duct. A single hose has the potential of lowering the air pressure and creating a vacuum. A two-duct system like Ecoflow’s, above, is more efficient with both air intake and exhaust, and more and more smart models are app-enabled. The downside of these AC units is their large presence in the room. Many look like rectangular robots. They’re heavy, but most have wheels to make moving them easier, along with the option to have the airflow go straight up. While the design of many of the units we’ve tested has gotten more compact and sleek, the ducts are still a lot. The wide tubing isn’t easy to hide. Still, if you need an AC and cannot use a window unit, portable air conditioners are a godsend.

What Are BTUs?

British thermal unit or BTU is the measurement of the energy required to remove heat within an hour. According to the US Department of Energy, you’ll need 20 BTUs for each square foot of living space. A 300-square-foot room will require an AC with 6,000 BTUs. It’s important to know your room’s square footage. A high BTU in a small room will result in improper dehumidification, and if the BTU is too weak, it will take longer for the unit to cool the room efficiently.

Looks

In the past, window units have been necessary eyesores. They were functional, but for years little had been done to make them aesthetically pleasing. That is no longer the case, with startups like July, which makes our pick above, reimagining what a window AC looks like. There are wood-panel fronts and sleek, modern designs that blend in with the room, and that trend isn’t letting up any time soon.