New Brunswick is under an Environment Canada heat warning, with the humidex expected to push temperatures into the 40-degree range, and the rest of the Maritimes is under a special heat alert.
In Fredericton, where a local level-1 heat alert has been issued, humidex levels — which take into account both temperature and humidity — are predicted to reach between 35 C and 39 C for much of the day.
"The combination of temperature and humidity is going to reach a level where people are at risk of heat-related illness," said regional medical officer of health Dr. Na-Koshie Lamptey.
She says the weather conditions could be hazardous for older adults, children ages four and under, people who are overweight and those with certain chronic medical conditions.
"There are several measures people can do to try and keep cool. One is to drink plenty of fluids; the other is to wear light, loose-fitting clothing and to try and use the shade when they're outside," said Lamptey.
"And we also ask that people check in on those who are more vulnerable to heat-related conditions to make sure they are doing OK."
According to Environment Canada, the temperature in P.E.I. is expected to hit the mid-30s Tuesday.
Organizers of Canada Day festivities at the P.E.I. 2014 Celebration Zone in Charlottetown say they've taken several measures to help crowds beat the heat.
"We made sure that we had water very accessible. We're pleased to have a quench buggy on site, where people can just get a little bit of water and that's all free and available to the public," said Penny Walsh-McGuire, P.E.I. 2014 executive director.
"At the Celebration Zone, there's lots of pavilions and tents to check out, which will give you a little bit of shade from the sun."
Organizers of the Charlottetown Event Grounds say free bottles of water will also be given to those waiting in line to enter the Big Red Festival concerts.
Environment Canada expects high temperatures and humidity will persist into Wednesday.