Nearly 200,000 people in Thailand have been admitted to hospital because of air pollution this week, officials have said, with Bangkok shrouded in a harmful haze.

The Thai capital, home to an estimated 11 million people and one of the world's most popular tourist destinations, has been blanketed for days by an unpleasant yellow-grey mix of vehicle fumes, industrial emissions and smoke from agricultural burning.

More than 1.3 million people have fallen sick in the kingdom since the start of the year as a result of air pollution, with nearly 200,000 admitted to hospital this week alone, according to the public health ministry.

Kriangkrai Namthaisong, a doctor at the ministry, Wednesday urged children and pregnant women to stay indoors.

Anyone going outside should wear a high-quality N95 anti-pollution mask, he added.

During another pollution peak in late January and early February, city authorities urged people to work from home.

A spokesman for Bangkok governor Chadchart Sittipunt -- elected last year with promises to improve the city's environment -- said they would not hesitate to issue another similar order if the situation got worse.

Aekvarunyoo Amrapala told AFP that nurseries run by the city had set up special "no dust rooms" with air purifiers to protect young children, as well as checkpoints to monitor vehicle emissions.

The public health ministry said 50 districts in Bangkok on Wednesday recorded unsafe levels of the most dangerous PM2.5 particles -- so tiny they can enter the bloodstream -- while Thursday they remained well above World Health Organization guidelines.

PM2.5 levels have been above safe limits for most of Bangkok for the past three days, according to the government's pollution control department.

The situation was worse in the northern city of Chiang Mai, in an agricultural region where farmers burn crop stubble at this time of year.

Around midday, the popular tourist destination was ranked the third-most polluted city in the world by monitoring firm IQAir.