People should consider using reusable masks instead of disposable ones to reduce waste, environmental group Greeners Action says.
Yet more than 90 percent of residents use disposable masks - a major danger.
The group's executive director, Angus Ho Hon-wai, said in a seminar yesterday that many used masks have been found in country parks.
"The disposable masks are made of plastic," he said, "and they are no different from plastic bottles."
He added that some masks also went into the sea via rivers and caused pollution.
And the micro-plastic produced by decomposing masks could enter the food chain after being eaten by fish.
Greeners Action project manager Yip Chui-man said studies show it takes 450 years for masks to decompose, which means a major risk for wildlife.
"Masks were found in dead penguins in Brazil," she said. "A crab was also found dead after being trapped in a mask in France."
Yip said interviews with more than 1,000 people last October showed 92 percent of Hongkongers using disposable masks.
"We also estimated that Hongkongers would use 54 million masks every week. This means the number of masks used by all Hongkongers since the pandemic started in February last year has exceeded three billion - as heavy as 1.4 million plastic bottles."
And more than 80 percent of people using disposable masks believe they are more hygienic, Yip said. But some reusable masks are just as effective in protecting people.
Greeners Action suggested authorities issue guidelines on using reusable masks while also recycling used disposable masks.
It also appealed to companies not to distribute disposable masks as gifts. Instead, they should give out reusable masks.