Senior citizens do morning exercises amid heavy fog in Hongze County, east China's Jiangsu Province, Jan. 14, 2013. [Xinhua]

Young pedestrians slapped on protective face masks while senior citizens carried on with their regular outdoor exercises as hazardous smog choked Beijing for a fourth consecutive day.

Beijing on Monday continued to issue air pollution warnings as readings of PM2.5 hit 300 to 400 micrograms per cubic meter in many parts of the city, or 12 to 16 times the level considered safe by the World Health Organization.

Officials and experts have advised residents, especially children and the elderly, to stay indoors. But on Monday, it was busy as usual on Beijing's streets, with people walking dogs and doing morning exercises.

Dozens of senior citizens did their morning exercises in a small park with exercise equipment near Xicheng District's Baiwanzhuang Avenue, and none of them were aware of the pollution readings or wore protective masks.

"I do have some coughing today, but I just don't want to be cooped up in my house or bother myself with an uncomfortable mask," said 63-year-old Peng Fuqiang, who regularly visits the park.

Like Peng, several middle-aged and elderly residents interviewed were nonchalant about the pollution. Many said they had gotten used to the city's many smoggy winter days, while others voiced suspicion over the severe health problems the pollution is said to trigger.

Doctors and health experts agree that excessive inhalation of PM2.5, or airborne particles smaller than 2.5 microns in diameter, raises people's risks for cardiac and respiratory diseases, with children and the elderly being the most at-risk groups.

In Xuanwu Park, located in a heavily populated part of Beijing, crowds of elderly residents played shadow boxing, jogged or participated in aerobic exercises.

Shao Xuexian, an 81-year-old asthmatic, was the only person there with a protective mask. He said he believes kicking the shuttlecock around is good for his health, so he does it even on smoggy days.

Other exercisers and park management staff said fewer people over 70 have visited the park in the previous three days, but residents in their 50s or 60s seemed largely undeterred by the smog.

Older Beijingers' apathy toward the unhealthy air runs a stark contrast to the conversation happening online. It seems like all Chinese netizens have turned to the Internet to complain about the choking air and criticize the slow progress the government is making in pollution control.