HONG KONG » Millions of Chinese, gripped and outraged, watched a 104-minute documentary video over the weekend that begins with a slight woman in jeans and a white blouse walking on to a stage dimly lit in blue. As an audience looks on somberly, the woman, Chai Jing, displays a graph of brown-red peaks with occasional troughs.

"This was the PM 2.5 curve for Beijing in January 2013, when there were 25 days of smog in that one month," Chai explains, referring to a widely used gauge of air pollution. Back then, she says, she paid little attention to the smog engulfing much of China, even as her work took her to places where the air was acrid with fumes and dust.

"But," she says with a pause, "when I returned to Beijing, I learned that I was pregnant."

She has said her concerns about what the filthy air would mean for her infant daughter’s health prompted her to produce the documentary, "Under the Dome." It was published online Saturday, and it swiftly inspired an unusually passionate eruption of public and mass media discussion.

"I’d never felt afraid of pollution before, and never wore a mask no matter where," Chai, 39, says in the video. "But when you carry a life in you, what she breathes, eats and drinks are all your responsibility, and then you feel the fear."

By early Monday morning, "Under the Dome" had been played more than 20 million times on Youku, a popular video-sharing site, and it was also being viewed widely on other sites.

Tens of thousands of viewers posted comments about the video, many of them parents who identified with Chai’s concern for her daughter. Some praised her for forthrightly condemning the industrial interests, energy conglomerates and bureaucratic hurdles that she says have obstructed stronger action against pollution. Others lamented that she was able to do so only after leaving her job with the state-run China Central Television.

"Support Chai Jing or those like her who stand up like this to speak the truth," said one of the comments on Youku. "In this messed-up country that’s devoid of law, cold-hearted, numb and arrogant, they’re like an eye-grabbing sign that shocks the soul."

Some have wondered how Chai got away with it.

China has been tightening restrictions on the Internet and the documentary is somewhat critical of the government.

Access to the video was not blocked, but by Sunday evening popular Chinese websites had removed prominent headlines and links about "Under the Dome" from their front pages, possibly at the behest of nervous propaganda officials.

Some officials, however, may even welcome it as an opportunity to build support for anti-smog measures. The website of People’s Daily, the main Communist Party newspaper, was one of the first to post the documentary. And the recently appointed minister of environmental protection, Chen Jining, praised the video.

"Chai Jing’s documentary calls for public environmental consciousness from the standpoint of public health," Chen said. "It deserves admiration."

© 2015 The New York Times Company