The hot and humid weather being felt across Toronto may not have anything on the sweltering conditions that many subway riders have been faced with so far this summer.

According to the TTC, between 20 and 25 per cent of the subway cars on the Bloor-Danforth line have been without air conditioning at any given time this summer due to mechanical issues.

The reason for the high number of malfunctioning air conditioning systems is two-fold, according to the TTC’s head of rail cars and shops Raffaele Trentadue.

First, the system is now 20-year-old and beginning to show its age and second there is only one air conditioning system on each of the trains on the Bloor-Danforth line, as opposed to two on each of the newer trains along the Yonge-University-Spadina line.

“It is a combination of things that have all come together at once and we are trying to avoid a band aid fix,” Trentadue told CTV News Toronto on Tuesday. “We don’t want the same cars coming back in one or two weeks with the same problem.”

Trentadue said that a maintenance program has been ongoing since this winter to identify subway trains with malfunctioning air conditioning systems and complete the necessary fixes.

The problem is that there is often a lengthy wait for replacement parts for the older system, Trentadue said, meaning that some trains are left without air conditioning for an extended period of time.

“We know how uncomfortable it can be so we are trying to mitigate this as much as possible,” he said. “We are limiting the number of cars in a six car train that have HVAC problems and operators are communicating with customers and letting them know that they can get off at the next stop and go to another car that does have air conditioning.”

In addition to working to fix malfunctioning air condition systems, Trentadue said the TTC is working to introduce better practices for the maintenance and cleaning of currently functioning systems as it works to stave off additional problems.

Unfortunately, no easy fix exists due to the age of the infrastructure.

“It is a little more challenging to set up fans in a subway,” Trentadue said.