How Hot Is Too Hot Inside a Home?

The limit of fan cooling

One number does establish a clear limit on a common response: CDC says to use fans only when the indoor temperature is below 90°F. At 90°F or above, do not rely on a fan for protection. Use air conditioning or move to a cooled place.

Act before the room reaches that point when someone is heat-sensitive, the room is still heating, the night will remain warm, cooling has failed, or leaving will take time.

Use the temperature as evidence, not a verdict

Use an existing thermostat, room thermometer, or other available indoor reading if you have one. Record the room, time, temperature, whether direct sun was entering, which cooling was running, and whether the room was still warming. A reading from one room does not describe the whole home.

You do not need to buy a special device before taking protective action. A device can be wrong, badly placed, or showing a cooler room than the one people occupy. How people are doing and whether the home can actually cool matter more than waiting for a perfect measurement.

The National Weather Service's HeatRisk forecast adds useful context. It combines daytime and nighttime heat, duration, time of year, and local health data. It describes outdoor heat risk over a 24-hour period; it does not measure the temperature inside the home.

Decide whether the home can provide relief

Staying is more reasonable when:

• at least one occupied room is cooling rather than continuing to heat;
• air conditioning or another effective cooling source is working;
• the people and animals who need the most protection can remain in that room;
• essential medicines and equipment can be kept within their required conditions;
• the household has water, power, communication, and a way to leave if conditions change; and
• overnight conditions will allow meaningful cooling.

Plan to use a cooled location when:

• the coolest room is approaching 90°F and still warming;
• the room is already 90°F or higher and the plan depends on a fan;
• the home has no effective cooling during a prolonged heat event;
• a power outage has stopped the cooling plan;
• poor outdoor air makes window cooling unsafe or ineffective;
• the home stays hot overnight; or
• a person, animal, medicine, or device requires conditions the home cannot maintain.

If anyone may have a heat-related illness, contact 911 or the local emergency number and follow the dispatcher's instructions. Do not use this guide to decide that symptoms are non-emergent.

What not to rely on

• Do not treat 90°F as a safe indoor limit. It is a fan-use boundary in current CDC guidance.
• Do not use the outdoor temperature, heat index, or HeatRisk color as if it were an indoor reading.
• Do not assume a basement, ground floor, or shaded room is cool enough without checking the actual space.
• Do not wait for a warning if the home or a person is already becoming too hot.
• Do not use an oven, stove, grill, generator, or fuel-burning appliance as part of an indoor cooling plan.
• Do not stay only to protect property, preserve a lease dispute, or avoid asking for help.

Next action

Choose the room that stays coolest. Record one cooled location the household can reach if that room cannot be kept cool enough, including its current source, hours, accessibility, transportation, and any animal or equipment restrictions.