A fishmonger uses battery-powered portable lamps in Yangon in Myanmar, where less than a third of the population have regular access to electricity

Myanmar's plans to grow the country's desperately needed but polluting coal-fired power plants could kill more than a quarter of a million people in the coming decades, environmentalists said Thursday.

The country's air is among the dirtiest in the world and pollution is only expected to worsen as the economy opens up after decades of isolation under the former junta.

A new study by Harvard University and Greenpeace warned that the government's plans to expand its current network of two coal-fired plants to 10 could have a major human toll.

Six of its cities already have higher counts of dangerous microscopic particles known as PM10 than China's famously smog-filled capital Beijing, according to 2016 data from the World Health Organization.

"These plans do not take into account the human health costs when making choices about the country's energy future," Lauri Millyvirta, from Greenpeace, said.

The extra pollution would likely cause more than 7,000 premature deaths a year, totalling 280,000 over the 40-year operating life of the eight new planned plants and the two operating ones, it predicted.

Half would be in Myanmar and the rest in neighbouring countries, mainly Thailand and China but also other parts of Southeast Asia, the study found.

The pollution would likely increase the risk of heart attacks, breathing problems and lung infections.

Myanmar has made coal-fired plants a cornerstone of a government plan to provide electricity to its entire population of more than 50 million people by 2030.

Less than a third of people have regular access to electricity through the country's dilapidated power grid, which frequently breaks down, and a lack of power is a major issue for attracting foreign investors.

While the country has abundant reserves of gas off its shores in the Andaman Sea, the vast majority is exported, mainly to Thailand and China.

Plans to build the $3.6 billion Myitsone hydroelectric dam on the mouth of Myanmar's Irrawaddy river, which would sell some 90 percent of its power to China, have also faced strong public opposition.

© 2017 AFP