The National Environment Board (NEB) will beef up preventive measures against PM2.5 ultrafine dust after 17 northern provinces have seen smog pollution hit crisis levels.
Siwaporn Rungsiyanon, a spokeswoman of the Centre for Air Pollution Mitigation (CAPM), said the 17 provinces have been seeing PM2.5 levels in excess of 100 microgrammes per cubic metre (µg/m3) since the middle of last month, as a result of slash-and-burn activities in nearby forests and farms.
The air there is considered far from safe as any PM2.5 level above 50 µg/m3 is considered unhealthy.
"The quality of air in the North is now at the highest alert, or the fourth level," Ms Siwaporn said.
"It is at a critical level. The NEB will tighten its air pollution control measures to deal with the problem, including strictly enforcing the law against anyone who sets forest fires or farmland burning. This is an urgent agenda the NEB will discuss during its meeting on March 15."
She said people living in the northern region should suspend outdoor activities until further notice.
Sensitive groups, including the elderly and children, should try to stay home as much as possible for the next five days, she said, adding they should stock up on food and medicine.
Ms Siwaporn also recommended schools consider closing temporarily to protect children's health given the toxic air situation -- although such decisions are at the discretion of the governor.
"Closing schools would be good for students' health, but that's not a solution to the problem. The source of the PM2.5 dust in the North is mainly from forest fires, not traffic problems like in Bangkok," she noted.
She said the Department of National Parks, Wildlife and Plant Conservation would close more national parks in the region, in an attempt to limit forest use in sensitive areas, including deploying more forest patrol teams to guard against illegal forest burning.