Several members of the Shinawatra family have fallen sick after a three-day trip to Chiang Mai last week, Paetongtarn Shinawatra said on Friday.

Everyone in the family seemed to have caught a cold while her son and daughter tested positive for Covid-19, said Ms Paetongtarn, the youngest daughter of former prime minister Thaksin Shinawatra and leader of the ruling Pheu Thai Party.

"I'm now afraid [that I might have Covid-19 as well], but my dad is feeling okay" Ms Paetongtarn said.

Her family's visit to Chiang Mai was when its haze pollution was amongst the worst in the world.

The number of sick people visiting Maharaj Nakorn Chiang Mai Hospital diagnosed with conditions linked to the PM2.5 pollution in Chiang Mai from Jan 1 until Mar 15 was 30,339 -- double that from the same period last year.

Although Chiang Mai's haze situation significantly improved after several days of rain, the PM2.5 level in certain areas remained high enough to be unhealthy, Chiang Mai's governor said.

The upper part of the province, which is closer to Myanmar, is heavily impacted by transboundary haze pollution, despite continuous efforts to curb the air pollution, including cloud seeding, which is now being carried out every day, he said.

Some 17 provinces, including Bangkok, on Friday recorded unhealthy levels of PM2.5 by 9am.

The serious air pollution situation, which persisted for weeks, has prompted Bangkok Airways to suspend flights to and from Mae Hong Son until March 30.