Those living in the most-polluted corners of the realm may live four years less than they may have elsewhere, according to a recent study.

Prolonged exposure to ultrafine particles called PM2.5 caused life expectancy of Thais to decrease by over two years on average compared to those living in areas that meet World Health Organization air quality standards, according to an analysis of conditions in 2016, which many worry has worsened since then.

On average, Bangkokians will live 2.4 years less, while people in Chiang Mai and Chiang Rai will respectively see 3.6 and 3.9 fewer summers, according to the Air Quality Life Index report published Wednesday by the University of Chicago. The index assesses the impact of particulate air pollution on life expectancy.

The report cited growth in the industrial sector and increased vehicle emissions for a 23-percent increase in air pollution since 1998, when it took 1.7 years off people’s lives.

It also found 87 percent of the population live in areas where pollution exceeded WHO guidelines. Even under more-forgiving Thai government standards, three out of four people breathe “unhealthy” air.

The report came out as the northern and northeastern provinces are choking on seasonal toxic smoke largely caused by open burning and forest fire. It followed severe air pollution over the capital in January which saw officials scramble to douse public furore with a series of ineffective measures.

According to Twitter, there were at least 4.8 million tweets about PM2.5 in Thailand sent out between December and January. Public pressure has led to more measures from the government including a ban on open burning, school closures and checkpoints to fine polluting drivers.

Another study by AirVisual, an independent monitoring organization, ranked Thailand the 23rd worst-polluted country in 2018 with average concentrations of PM2.5, the smallest and most harmful particles, at 26.4 micrograms per cubic meter.

The WHO’s target is 10 micrograms per cubic meter.

While Bangkok was the world’s 24th most-polluted capital last year, Thailand’s foulest provinces were Samut Sakhon, Nakhon Ratchasima and Nong Khai.