Smog crisis in Northern Thailand starts in the dry season from February to
April of every year, especially in March, of which the problem becomes most serious.
During this period, the Northern region is covered by fine particulate matter smaller
than 10 microns (PM10) higher than safety standard, i.e. 120 milligram per cubic
metre within 24 hours. Data shows that during the dry season of the past several
years, level of particulate matter smaller than 10 microns reached the level that is
harmful to people’s health as well as the environment as shown in the following table