Ottawa Race Weekend organizers have changed the start times for two of its races this weekend to try to avoid the overwhelming heat and high humidity.

Saturday's 10 kilometre race will start at 7 p.m. instead of 6:30 p.m., while Sunday's half marathon will start at 8:15 a.m. instead of 9 a.m.

The full marathon will still start at 7 a.m. on Sunday.

Environment Canada predicts Sunday's temperature will reach a high of 31 C.

Organizers say they could still stop the races if medical operations become overwhelmed.

We've had discussions with our medical team, logistics team, and partners at the city/police to make sure that we can put on a safe event. —@OttawaMarathon "We urge runners to co-operate with us and hydrate frequently during their race, adjust their pace as required, and pay close attention to the race crew and the weather warning flags posted along the race route," event director John Halvorsen said in a statement.

"If runners co-operate fully, we will reduce the chances of overloading our medical operations, which are a critical requirement for safe race events. This plays an important role in making decisions about proceeding with or stopping [races]."

Ottawa paramedics are urging runners to write emergency contact information and a list of what medications they take on the back of their race bibs in case something happens.

An important message to all runners this weekend: please fill out your race bib. <a href="https://t.co/uZwF1jwt85">https://t.co/uZwF1jwt85</a> <a href="https://twitter.com/OttawaMarathon">@OttawaMarathon</a> <a href="https://twitter.com/runottawa">@runottawa</a> —@ottawaparamedic "The calls we get and the cardiac arrests that happen are mostly because people ignored and kept pushing because [they think] 'Oh my goodness, I'm only two, three, four kilometres away from the finish line, I'm going to keep going.' That's when they collapse," paramedic J.P. Trottier said.

To battle the heat, organizers have added misting stations along the 10K, half-marathon and marathon routes. They're also bringing in more ice and cups for the water stations.

Ottawa paramedics said runners need to listen to their bodies and slow down or cool off during the race if they're not feeling well.

Four buses will be set up at medical stations on the two longer routes so runners can take a break in an air-conditioned space.

Organizers are even asking non-runners to pitch in. People along the courses are encouraged to run their hoses, sprinklers and have extra water and refreshments ready.

What to do

The race's medical director, Dr. Jon Hooper, has posted advice on avoiding dehydration and heat illness when running in hot weather on the event's website:

Pour water on your head to cool yourself down during the race (along with drinking water).

Drink ice-cold water and pour some ice-cold water on yourself right before the race.

Check with your doctor if you're taking medication as overheating can be a side effect.

Don't try to reach a personal best.

Drink more fluids than you usually do.

Wear lightweight, light-coloured clothes and a visor if possible.