An independent panel named by Ontario's Natural Resources Ministry to investigate the province's use of Agent Orange has concluded workers with three government agencies were exposed to unsafe levels of the herbicide.

The panel was announced by the government in 2011 after former forestry and hydro workers said they had been exposed to the toxic herbicide 2,4,5-T — a dioxin-laced component of Agent Orange.

Chaired by Dr. Leonard Ritter, a University of Guelph toxicology professor and expert, the five-member expert panel was handed more than 45,000 government files in some way linked to Agent Orange.

Ritter said the panel concluded that Agent Orange exposure "substantially exceeded safe levels" for Ontario Hydro, the Ministry of Natural Resources and the Ministry of Transportation workers involved in mixing, loading and applying the herbicide.

The number of workers would have been in the hundreds, and "often it was the same groups that applied across various sites," said Ritter, adding that applications by the three agencies accounted for roughly 90 per cent of cases.

The herbicide was used by Ontario hydro officials to clear power line corridors across the province.

"Some of those populations exceeded the [exposure] thresholds that we had established by six or seven hundred fold," he said.

The government has said it was clear Agent Orange was "very widely used" for more than three decades.

"We are sincerely sorry that some workers may have been exposed to 2,4,5-T in the years leading up to 1980," said Ontario Minister of Natural Resources David Orazietti on Thursday.

Model assessments

All of the panel's predicted exposure levels were not based on data on historical levels, because none was ever collected, but model assessments created by the panel.

"We may have overestimated some risks, but we’re confident we did not underestimate them," Ritter said, noting that the models cannot definitively determine if exposure led to increased health risks.

The herbicide 2,4,5-T wasn't banned by the Ontario government until 1979, and six years later by the federal government.

Ritter said the highest levels of Agent Orange were used by municipalities, rather than the provinces and its agencies, but the panel did not study those results.

Orazietti said the government would be reviewing the report and examining "what additional steps we can take."