There have also been renewed calls for a population health study into the cumulative effects of mining and power station pollutants.

The Upper Hunter Air Quality network's Muswellbrook monitor issued an alert for size 2.5 particulate matter early on Sunday morning.

It follows several last month from other monitors in the network for particulate matter concentrations that exceeded size 10 and 2.5.

The Department of Environment and Heritage yesterday blamed wood heater smoke for triggering the latest alert.

But the Singleton Healthy Environment Group questioned whether wood smoke was being used to deflect attention away from the issue of pollution from mines and power stations.

"It's not much of an excuse to say it's just wood smoke when they know there are major pollution issues associated with mines and power stations," group spokeswoman Carol Russell said.

Ms Russell said the number of alerts reinforced the need for a broader population health study in the region.

The group has also renewed its calls for the network to monitor a broader range of pollutants.

"There's not doubt that the network is a good thing but it is not monitoring other pollutants that we know are in our airshed," she said.

Fourteen monitoring sites make up the Upper Hunter Air Quality Monitoring Network.

A Department of Environment and Heritage spokeswoman said further monitors had not been ruled out.

"The Office of Environment and Heritage has always been aware that in order for the network to remain effective in monitoring regional air quality that current site locations and/or the establishment of new air monitoring locations will need to be regularly reviewed and considered with advice and input from the network advisory committee," she said.