Members of CUPE Local 416 set up a picket line outside a water treatment facility in Toronto's west-end on Monday morning after they went out on strike to back demands for a new contract with the city. (Lorenda Reddekopp/CBC)

Likely the most contentious issue in the Toronto city workers strike — certainly the one that has garnered the lion's share of the public's ire — is the ability of the CUPE workers to bank unused sick leave.

Under their last contract, unionized workers are allowed to store up a certain number of days, which they can then "cash out" when they retire.

It's supposed to be an incentive not to take sick days.

The thinking is that employees who are entitled to sick days that evaporate every month will just take them, whereas employees entitled to bank their sick days will only take days off when they are really sick, and save the rest.

Elementary Teachers of Toronto are another of the many groups of unionized workers with the ability to bank unused sick days and receive a payout upon retirement. "Then the teacher will receive half a year's salary as a final pay-out," says a statement on the ETT website.

The Canadian Union of Public Employees workers on strike in Toronto earn 1.5 sick days every month. If they don't use the days, they go into the bank. That could amount to 18 days a year.

When he or she retires, a unionized worker with 25 year's experience can cash those days out for a maximum of a six-month bonus.

Critics say Toronto's problem is that it hasn't set aside enough money to pay for those future benefits.

"It's $250 million [liability] for the City of Toronto," said Coun. Doug Holyday. "We simply can't afford that any longer."

Holyday dealt with the exactly the same issue when he was mayor of Etobicoke in 1996.

After Etobicoke contracted out its garbage service it struck a deal with its union to end the banking of sick days.

In 2000 York Region did the same thing with its ambulance workers.

Holyday says Toronto should never have let it get to this stage.

"It is a matter of coming up with a formula to buy out employees of their sick pay. A lot of employees would like to do that," he said.

For its part, the union says it's about fairness.

"This is about getting a fair deal similar to what everyone else got," CUPE Local 79 president Ann Dembinski said earlier this week. "Everyone else was able to negotiate a collective agreement without huge takeaways. These are huge concessions. No other City of Toronto workforce has had to negotiate any concessions in order to get a collective agreement."

CUPE officials point out that Toronto's firefighters and police officers got to keep their leave banks in recent contracts.

Some who have studied the issue, however, say that's an unfair comparison.

"Firefighters and police officers don't have the right to strike," said former Winnipeg mayor Glen Murray, who now works for the Canadian Urban Institute in Toronto.

Strikes have been fought over the issue.

Just last year, Public Service Alliance of Canada workers walked off the job at Canada Post when management wanted to wind down their leave bank.

The two sides worked out an agreement and settled their dispute.

The two CUPE locals involved in the Toronto strike say the sick-leave bank issue is just one of dozens of concessions the city is after.

They're also concerned about other proposals that they say would erode seniority rights.