In Masterton, the number of high air pollution (PM10) days per winter has fluctuated from year to year since monitoring began in 2003, with a dramatic reduction observed in 2015. Although emissions from domestic heating have been declining in recent years, the strong El Niño during winter 2015 resulted in a windier than normal conditions and this was primarily responsible for the low number PM10 exceedances of the National Environmental Standard for Air Quality (NES-AQ) in 2015.