The New South Wales Office of Environment and Heritage issued a health alert saying one of Singleton’s two air monitors had recorded a 71.5 PM10 reading at 7am.

PM10 is a description of fine particles suspended in air that damage people’s respiratory tracts and a 71.5 reading is in the upper reaches of the environment department’s “poor” air quality scale which runs from 51 to 75.

The air monitor, north west of Singleton, registered hourly air pollution readings of 110.8 at 2am, 124.6 at 3am, 62.5 at 4am, 113 at 5am and 126.2 at 6pm.

The Office of Environment and Heritage atmospheric sites manager Chris Eiser said the hourly peaks could not be described as “hazardous”, as the department’s scale indicated, because they should be averaged over a 24 hour period.

Mr Eiser said he could not say what caused the air to fail national health standards as there could be a variety of contributing factors, such as open-cut coalmining, agriculture, motor vehicle emissions and fires.