BANGKOK - There are two pictures Thai air pollution expert Supat Wangwongwatana likes to show whenever he talks about Bangkok's transition, in a little over a decade, from a city blanketed in smog to one boasting clear blue skies.

The first, taken in the mid-1990s, shows the skyline of downtown Bangkok. Most buildings are in silhouette, shrouded in a thick layer of haze. The Baiyoke Tower, back then the city's tallest building, was still under construction.

The second, taken a decade later from a similar angle, looks dramatically different. The sky is blue, the clouds are visible, and the buildings bathed in sunlight.

"Sulphur in diesel fuel and gasoline back in the early 1990s was 10,000 parts per million. Today, it is less than 50 ppm," said Supat, referring to a natural component of crude oil that contributes to air pollution.

"That early technology enabled us to reduce emissions," he said, citing processes for producing cleaner fuel and catalytic converters that make exhaust pipe fumes less toxic.

Bangkok, home to some 9 million people, remains relatively smog-free, even though vehicle numbers have been increasing every year, experts say. In the first four months of 2017, the city registered 300,000 new vehicles, including buses and motorcycles, bringing the total to nearly 9.5 million.

The World Health Organization calls air pollution "a public health emergency". An estimated 6.5 million deaths, nearly 12 percent of all global deaths, were associated with indoor and outdoor air pollution in 2012, the bulk in the Western Pacific and Southeast Asian regions, WHO data shows.

"If nothing was done at all during the last 20, 25 years, I cannot imagine what Bangkok would be like now. People would probably be sick from air pollution," Supat said.

Despite having the second-worst traffic congestion in the world after Mexico City, according to a global traffic index compiled by navigation company TomTom, Bangkok topped a 2015 list ranking popular tourist destinations on their air quality, from United Kingdom-based firm Airport Parking and Hotels.

It may not stay there for long.

"Since the 1990s, the number of automobiles is increasing, so you have more congestion and more sources of emission. That's a big challenge," said Bhichit Rattakul, who founded the Anti-Air Pollution and Environmental Protection Foundation a decade before being elected governor of Bangkok in 1996.

To combat this, the PCD has reached preliminary agreements to impose the Euro 5 standard, which further limits pollutants in fuel, by 2023 for oil refineries and by 2024 for vehicles.

"The important thing is that the Thai government implements these policies with Thai industry," said Teera Prasongchan, chair of the Thai Automotive Industry Association's committee on technical issues and deputy general manager of Toyota Motor Thailand. "That means we have to sit down and talk, and compromise with each other."

Reuters