TORONTO -- Ontario is bringing in new rules to help prevent glass panels from breaking off highrise balconies and tumbling onto sidewalks below, but isn't forcing the safety upgrade on all buildings.

The revision to the province's building code means construction projects started after July 1 will have to use heat-strengthened laminated glass -- the same kind used in car windshields -- for panels on the outer edge of balconies.

However, existing highrises and condominiums as well as those currently under construction won't need to fall in line with the changes.

"The reality is that if a (building) permit has already been issued they don't have to abide by the new amendment in the building code," Municipal Affairs and Housing Minister Kathleen Wynne said while announcing the rules Thursday.

"But I believe that this will send a very clear message to builders about how high we think the standards should be."

The changes come after a series of incidents over the last year in Toronto's downtown core that have seen glass panels burst, sending shards from condominium and highrise balconies onto streets below.

The revisions are an interim measure until national guidelines dealing with balcony glass are completed in the next two years, Wynne said.

The change comes as Toronto's downtown core leads North America in active condo developments. There are 173 condo towers under construction in Toronto, with another 325 in the works, a real estate conference in the city heard earlier this month.

Impurity "inclusions" of nickel sulfide in tempered glass commonly installed for balcony panels on the sky-high buildings are likely behind the shattering incidents, said Doug Perovic, a material science and engineering professor at the University of Toronto.

He said the impurities in the glass grow "like a cancer," creating stress which can push glass to the bursting point.

Perovic said the new rules will work well to keep panels from splintering outward when glass shatters since the laminate will hold the shards in place.

But he said panels which have been installed in the last two to three years should all be reviewed to see if the problem stems from a handful of suppliers churning out bad batches of glass -- which could shed light on which buildings may have flawed panels.

Glass already installed would have to be broken to test for the flaw, he noted.

The province should look into "where the glass is being sourced and whether there is a systemic problem with one or more of these suppliers," Perovic said.

"We still may have bad panels that are sitting out there waiting to spontaneously fracture that were put in over the last couple of years."

"While we've done something good going forward, we leave the public at risk in terms of the potential for many of these glass balconies to shatter," said Rosario Marchese, an NDP member of provincial parliament who represents a downtown Toronto riding.

Marchese says he's counted around 30 cases of falling glass from condos and highrises in Toronto in the last two years. In March, two $20-million class action lawsuits were filed against a pair of developers by condo dwellers over the issue.

Last year's high-profile string of shattered panel incidents has led to a shift in how developers think about balcony glass, said David De Rose, an executive with engineering consultant Halsall Associates.

As a result "there's a lot more discussion, a lot more scrutiny in terms of the way things are going to get designed, the way things are going to get built," he said.