A dispute is heating up between unions representing federal workers and the government over numbers the Conservatives are using to rationalize changes to health and safety laws.

The Public Service Alliance of Canada has accused senior officials with Employment and Social Development Canada's labour program of presenting misleading numbers to a committee of MPs studying proposed changes to the Canada Labour Code.

PSAC said the numbers presented at last week's meeting were wrong and don't tell the whole story.

"Any closer scrutiny of the numbers does not support what they're intending to do," said Bob Kingston, president of the Agriculture Union with PSAC. "So I'd have to let other people draw conclusions about whether they intended to obfuscate or whether that's by accident."

At the heart of the dispute is the number, often quoted by government officials and the labour minister, that in 80 per cent of cases where a worker has refused work due to danger, an investigation and, in some cases an appeal, has made a finding of no danger.

This number appears to be the basis for the impetus to make changes to the legal definition of danger and to the process by which federal employees can refuse unsafe work.

"The members of Parliament were not misled. The information we provided was accurate," said Employment and Social Development Canada acting director General Brenda Baxter in an interview with CBC News.

But opposition MPs on the human resources, skills and social development committee felt the numbers were incomplete. They asked government officials whether, in those 80 per cent of cases, health and safety officers investigating took other enforcement action even if an actual finding of no danger was made.

The officials said they would have to go back and look through the data manually to determine that.

"If they are so slipshod that they are proposing these huge legislative changes without even taking the time to look at the numbers to support their proposals, that's crazy," Kingston said in an interview with CBC News.

Kingston said PSAC has already done the analysis the department said it had to do manually. The union crunched the numbers of a random sampling of 40 per cent of the more than 1,000 cases of unsafe work refusal investigations over the past 10 years.

Its analysis found that in 52 per cent of group refusals and 45 per cent of individual refusals there was some enforcement action.

Baxter said her department is trying to get its own analysis done before MPs vote on the legislation.

"We're working earnestly and putting in a great effort to respond to the questions raised by the parliamentarians," she said.

The changes to the Canada Labour Code are included in Bill C-4, the federal government's omnibus budget bill. The code covers about 1.5 million federal employees in sectors such as railways, shipping, pipelines and mining.

The legislation would change the definition of danger from the broad idea of a potential hazard to something that poses an imminent or serious threat to life.

One expert agrees unions have reason to worry about that.

"There's no doubt the threshold test that an employee is going to have to meet to justify a continuing refusal to work will be much higher than it is now. The spectrum of what constitutes a danger in the workplace is going to be significantly narrowed," said Ronald Snyder, a specialist labour employment lawyer in an interview with CBC News.

Snyder said a lot will depend on how Labour Minister Kellie Leitch and her staff interpret the definition when cases come forward. But he said regardless, the number of complaints will go down under the proposed definition, as will the number of decisions in employees' favour.

Another major change would see the minister have the power to decide, without an investigation, that a work refusal complaint is frivolous or vexatious and dismiss it. The employee could then be disciplined.

But Snyder said this change is a positive one as it will help cut down on frivolous complaints, which he said can be a problem, especially in the public service.

Kingston said frivolous complaints are not a rampant problem. Kingston co-chairs with the Treasury Board, the public service policy committee on health and safety, which looks at issues government-wide.

"If this was really an issue, then why was it never brought forward? It's never been raised," he said, adding he knows of dozens of cases where people should have refused work but didn't.