This is what has been posted on their Air Canada website in regards to your question:
If you are buying a non-refundable ticket, please be advised that, beyond 24 hours of purchase*, Air Canada cannot make any exception to the fare rules in the event that an unexpected medical emergency or other unforeseen circumstances may force you to cancel your trip or modify your travel plans. It therefore remains your responsibility to purchase cancellation insurance to guard against such risks.
Behind the facts:
A non-refundable ticket offers no options in the event that an unexpected medical emergency or other unforeseen circumstance forces you to cancel your trip or modify your travel plans.
The best advice here is to prepare and plan for the unexpected. Many travellers consider the purchase of cancellation insurance as a good investment against such risks. Some payment cards also cover trip cancellations; check with your bank or service provider. The time to make these decisions is when you purchase your ticket. RBC Travel Insurance is available via aircanada.com should you wish to protect your ticket's value.
*Air Canada will cancel any purchased ticket and provide a full refund without penalty up to 24 hours after purchase.