The plants are 60-200 kilometers to the east and a southeasterly wind is blowing now bringing clean air from the sea and creates good diffusion, it said in a statement. It rejected statements by an environmental activist that the coal plants are polluting the city.

The company said the capital is being affected by high dust levels whereas other northern cities and provinces, including those nearer the plants, do not have similar high levels of dust.

EVN pointed to an explanation by the Hanoi Environmental Protection Department saying on polluted days the city has unfavorable meteorological conditions.

It is sunny during the day, heat levels are higher than during nights, smog covers the entire city in the early mornings, and wind speeds are low.

Virtually every aspect of life in Hanoi has been disrupted by the pollution.

The poor air quality in Hanoi and Saigon has made headlines since September. In October the capital named the sources of its pollution as traffic emissions, domestic cooking coal, construction sites, and animal farms.

With records toppling in 2019, the capital’s air quality index (AQI), which shows how polluted the air is, went past 200 for four consecutive days starting on November 7 before topping 300 on November 12, according to the Department of Natural Resources and Environment. An AQI level above 100 is considered unhealthy for humans.

Not until December did the Ministry of Health issue a 14-step guideline to help deal with air pollution, including close supervision of air quality, use of masks and improving personal hygiene.

EVN said it has conducted environmental impact assessment of its plants. It found that those within a distance of 2.6 – 2.8 kilometers cause the largest dust concentration, and the further away the plants, the less the dust concentration; at a distance of 5-10 km or more the dust almost does not affect the city.

EVN operates several coal-fired power plants in the north and others situated 200 – 300 kilometers from Ho Chi Minh City in the south.

"In order to ensure the minimum concentration of dust emissions into the environment, EVN has installed electrostatic precipitator systems with a dust removal rate of over 99.6 percent at all its coal-fired plants," the company said.

The dust levels recorded between the beginning of September and October 5 at the plants were low according to the standard set by law and similar to data collected there three to six months earlier, it said.

In contrast, Hanoi consistently suffered high levels of PM2.5 -- described as superfine particles measuring 3 percent the diameter of a human hair – of above 50 μg/m3 for 19 days between September 12 and 30.

This was the worst in five years, according to a report from the Ministry of Natural Resources and Environment.

Vietnamese standards prescribe safe PM2.5 limits of 50 µg/m3 for 24 hours and 25 µg/m3 on average over a year.

The World Health Organization’s limits are 25 and 10.

Tran Dinh Sinh, deputy director of the Green Innovation and Development Centre (Green ID), had told the media earlier that studies indicated that one of the sources of PM 2.5 in Hanoi was cement factories and coal-fired plants in its vicinity.

"Dust from the factories and plants is brought to Hanoi by wind. There are air masses which when moving can travel thousands of kilometers, bringing dust with them."

Sinh said Green ID has invited a number of foreign experts to assist them in calculating the movement trajectory of PM 2.5 dust and there is evidence that dust travels from industrial zones in Hai Phong City and Quang Ninh Province where many power and cement plants are situated.

However, GreenID has not been able to calculate the effect of each individual plant, he said.

By 2030 Vietnam's premature mortality from PM2.5 emitted by coal-fired power plants is predicted to be around 19,220 per year, a 4.5-fold increase from 2011, a 2017 study authored by Harvard researchers into diseases caused by coal-fired power plants in Southeast Asia said.

Hanoi suffers extreme air pollution for around 300 days a year, with over 60,000 related deaths, a 2016 report by the WHO stated. 

Vietnam relies mostly on hydropower and thermal power.