(AFP) - Toxic smog forced hundreds of Bangkok schools to close Wednesday, as authorities struggle to manage a pollution crisis that has stirred widespread health fears and taken on a political edge just weeks before elections.

The Thai capital has been shrouded in murky haze for weeks, sparking social media criticism of the uneven response by the government and prompting rare scenes of residents donning masks on streets and on public transport.

Reasons given for the lingering pall include exhaust from traffic, unfettered construction, the burning of crop stubble, and pollution from factories getting trapped in the city.

Authorities have seeded clouds to provoke rain, sprayed overpasses with water to catch micro-pollutants and even asked people not to burn incense sticks and paper during Chinese New Year celebrations.

The measures so far have drawn derision from many Bangkok residents, while stocks of pollution masks have run out in many shops.

But on Wednesday, the Bangkok Metropolitan Administration stepped up its health warnings, ordering all 437 city-controlled public schools to close from lunchtime through Friday, designating 1,500 square kilometres (580 square miles) of the city a "control area".

"The situation will be bad until February 3 to 4, so I decided to close schools," said Bangkok governor Aswin Kwanmuang, adding he hoped the move would also empty the road of cars on the school run.

Three to four of the city's districts are "severely hit with smog", he added.

A spokesperson for the prime minister's office said private and vocational schools would be closed as well, but did not give a number.

- 'Public health crisis'-

At a downtown Bangkok school where parents arrived early to pick up their children, pupils said they knew about the risks posed by the dangerous pollutant particles, known as PM 2.5.

"I use a mask wherever I go," said 12-year-old Chaiwawut Benpalee. "It will not affect us now, but it will in the future."

Fleets of drones are set to be deployed to disperse sugary liquid solution to help clear the air of microscopic particles.

It is not clear how effective that will be given the scale of the smog cloaking the city.

Aswin also said City Hall may soon issue a warning against exercising in parks.

Air Visual, an independent online air quality index (AQI) monitor, on Wednesday pegged Bangkok at the "unhealthy" level of 171, up from 156 mid-month.

"It's a public health crisis," said Tara Buakamsri, Greenpeace country director for Thailand.

Measurements of harmful particulates are higher than some cities in China but well below the Indian capital New Delhi.

Siwatt Pongpiachan, a professor of environmental science at the National Institute of Development Administration (NIDA), said that while the cold dry climate was part of the problem the government should "think seriously" about congestion measures limiting the number of cars on the road.

- Pollution politics -

The battle against smog comes during Thailand's key tourist season, with the Chinese New Year getaway looming, in a country where earnings from foreign visitors make up around a fifth of the economy.

Edward Huang, a 28-year-old tourist from Taiwan who was wearing a mask, said visitors may be deterred from trips to Bangkok.

"The air is just so bad," he told AFP.

The haze is also starting to seep into the political conversation.

Thailand is set to hold an election on March 24, the first since the military took over in a 2014 coup.

"I don't see serious measures being taken," tweeted Chaturon Chaisang, a key member of the Thai Raksa Chart party, urging more to be done beyond the school closures.

His party, closely linked to the politically powerful Shinawatra clan, has been quick to launch a "measure the pollution" app, while other parties have handed out masks -- leaving the ruling junta appearing flat-footed in its response.

© Agence France-Presse