Dust levels rose by about 10 per cent whenever a coal train went past, the NSW Environment Protection Authority told a parliamentary hearing in Newcastle on Monday.
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But this was not evidence to support covering coal wagons because non-coal freight trains caused a similar increase, indicating that emissions from diesel locomotives were probably to blame.
EPA chief executive Barry Buffier was one of a number of EPA and Office of Environment and Heritage officials to give evidence at Newcastle City Hall to an upper house committee inquiring into the EPA's performance.
The investigation began in June and runs until February 14, with the seven committee members examining the EPA's response to six issues, including its "investigations and public statements about the effects of coal dust pollution in the Hunter".
The inquiry also took evidence from the NSW Minerals Council, the Coal Terminal Action Group and Hunter Councils.
Minerals council industry and environment director David Frith said the Hunter's air quality had been stable over the long term despite a substantial expansion of coalmining.
Mr Frith said smaller MP2.5 particles were a greater risk to human health than the larger PM10 particles and the latest evidence showed coal was "not a significant contributor" to this type of particulate pollution.
He said there was a perception in the coal industry that it was being unfairly targeted by the EPA, and was taking a more "heavy handed approach" to regulation.
But if the minerals council thought the EPA was being too hard on coal, the community activist members of the coal terminal action group argued it was far too soft.
John Mackenzie, James Whelan and Nick Higginbotham attacked the EPA over a coal train dust study that had most of its conclusions changed – including three that were reversed – in the days before its release in May last year.
The EPA had earlier defended its methods on the study when questioned by committee members including Mehreen Faruqi of the Greens and Luke Foley from the ALP, who both accused it of ignoring some of its own experts.
Dr Mackenzie said that as a result of the doubts over the various rail dust studies there was "no consensus view" of the contribution of coal trains to air quality, and therefore no way to evaluate the quality of the EPA's response.
