Temperatures near 30 degrees Celsius are expected across parts of southern and central New Brunswick on Wednesday and Thursday.
Environment Canada issued the heat warning and said that with the humidity, it could feel closer to 37 degrees.
Overnight temperatures are forecast to remain between 17 and 20 degrees, offering little relief from the heat.
Cooler air is expected Thursday evening, especially along the coast.
The heat warning issued covers several regions:
- Moncton and Southeast New Brunswick
- St. Stephen and northern Charlotte County
- Sussex and the Kennebecasis Valley
- Kent County
- Fredericton
- Oromocto
- Grand Lake
- Kouchibouguac National Park
According to Environment Canada, the heat may pose health risks, particularly for older adults, young children and people with certain medical conditions.
The agency said people should watch for early signs of heat exhaustion, which can include headache, dizziness, nausea, extreme fatigue and dark-coloured urine.
Health officials warn that heat stroke is a medical emergency. Symptoms may include confusion, red or hot skin, or fainting.
If someone shows signs of heat stroke, people are advised to call 911 immediately.
While waiting for help, the person should be moved to a cooler place, with excess clothing removed, and cold water or ice packs applied to help lower their body temperature.
To reduce the risk of heat-related illness, Environment Canada recommends drinking water regularly, even before feeling thirsty.
People are also encouraged to use fans or air conditioning if available, keep blinds or curtains closed during the day, and spend time in cooler public places such as libraries, community centres or shaded parks.
Public health officials also advise checking in on neighbours, especially seniors or those living alone, during periods of extreme heat.