The power plant is located 57km from Song Khwer district and 110km from Muang district in Nan, making the residents vulnerable to health problems from pollution fallout, they say.
Narongrit Siriwongworapat, a member of the Watershed Conservation Network based in Song Khwer district, said Nan residents fear they will suffer health problems when the plant comes online in two years.
The Hongsa plant is being constructed by Hongsa Power Company Ltd (HPC) formed by the Lao government and private shareholders, mostly Thais, in 2009. It is a lignite-fired plant which will send most of its electricity to Thailand from 2015.
It will become Laos' largest coal-fired plant, producing 1,878 megawatts, of which 1,500 megawatts will be exported to Thailand.
"Nan province is facing a severe level of air pollution at the moment. We're suffering haze problems caused by forest fires," Mr Narongrit said.
"But if the province has to encounter pollution from the Hongsa coal power plant, of course the air will become a lot worse."
He said Thailand has already learned a lesson from Lampang's Mae Moh coal power plant which discharged air pollutants, causing many health problems for people living near the plant. He said he did not want to see a recurrence of the problem.
Renu Vejaratpimol, a lecturer at Silpakorn University's Faculty of Sciences, who is closely monitoring the construction of the Hongsa plant, said when it comes online, it will definitely create air pollution as lignite is a poor quality coal.
"Even though it is still unclear how much greenhouse gas and particles will be emitted from the plant, we should take a lesson from Mae Moh power plant," Ms Renu said.
She said Mae Moh discharged 5.08 grams of nitrogen dioxide per kilowatt/hour, 5.27 grams of sulfur dioxide and 0.62 grams of particles. In addition, the ash from the coal contained heavy metals which cause cancer.
"It is difficult to prevent trans-boundary pollution as Nan province has already lost trees which act as a buffer zone. I think the best way to prevent the problem is to plant more trees to help absorb pollutants," she said.
Sathaporn Somsak, an advisor to the Nan-based Natural Resources and Environment Protection Network, said residents of Song Khwer district also feared an impact from a high-voltage cable line from the Mae Moh plant in Lampang to the Hongsa plant which will run through the district's forest areas.
Banpu Public Company Ltd, a major shareholder in the Hongsa plant, has not commented on the claims.
