Although particle levels have declined substantially since the 1970s, particle pollution has shown no discernible trend over the last decade. The Air NEPM 1-day standard for PM10 is 50 µg/m3 and is not to be exceeded more than five times a year at any one site. The number of allowed exceedences takes into account bushfires and similar natural events and necessary bushfire hazard reduction burning. In 2001, exceedences occurred on 8 days in Sydney and 5 days in both the Lower Hunter and the Illawarra. Bushfires are responsible for most occurrences of high levels of particulate pollution. Occasionally, widespread dust.
