A sweltering new trend has taken over parks across Shanghai this summer — the rise of the back sunbathing group. These are groups of people who bare their backs to soak up the sunlight, believing the practice expels dampness and toxins from the body.

On a recent afternoon, Zhongshan Park was dotted with several people sprawled out on mats, shielding their faces with umbrellas as they turned their backs toward the sun.

"I always stay in air-conditioned rooms, and like consuming cold drinks and spicy foods. Back sunbathing helps expel the internal dampness," explained 26-year-old devotee Zhang Xiao, who said this has helped her feel more energized.

"At first I didn't sweat, but later the pores opened up and I could feel the moisture beading up and trickling down. I had intended to listen to music, but then discovered that taking in the natural sounds of the park is far more relaxing," she said, adding the hooded protection allowed discreet sunbathing.

On social platforms such as Xiaohongshu, the back sunbathing topic has gone viral, with popular sites such as Century Park and Zhongshan Park in Shanghai frequently highlighted for their scenic lawns. Groups also proactively organize sunbathing yoga meetups.

However, the displays have drawn objections from some park visitors. Security guards have reported a recent spike in sunbathing groups during the dog days of summer, especially in the morning and late afternoon periods.

One guard has even seen some go topless as they sunbathe.

"If we see them removing all their clothes, we have to intervene and stop it," the guard said.

Park officials find themselves caught in the middle. While Shanghai's park regulations ban shirtless activities, there are no explicit rules prohibiting sunbathing, leaving policies open to interpretation.

"We try to strike a balance," Hu Yucheng, director of Shanghai's Heping Park, told media outlet Xinmin Evening News, adding that he was "exhausted" by the issue.

"Visitors who come to the park for back sunbathing and expose too much skin have often been complained about by other tourists. Yet after the park staff spoke to them, these sunbathers would then lodge complaints against the park," Hu explained.

Health experts, meanwhile, urge caution regarding the risks of excessive back sunbathing.

"Moderation is key, as with any therapy," advised doctor Shi Xiangdong of Huayang Community Health Service Center.

Around 20 to 60 minutes of back exposure can help absorb invigorating sunlight and disperse internal cold and dampness, said Shi, noting that as yang energy is believed to be stored above the hip, gently exposing this area allows the sunlight to permeate the whole body.

Shi stresses that proper back sunbathing methods are crucial. "It is more appropriate to wear clothing while exposing the back areas. And never directly expose the head, which has excessive yang energy. Use a hat or umbrella for shade."

He also cautioned against sunbathing for too long, which could lead to fatigue or profuse sweating that depletes vital bodily fluids.

"The key is achieving a mild, comfortable sweat," he said. "If you feel unwell, that's your body's signal to stop."

Proper timing and moderation are also essential, Shi noted. For the elderly, he recommended only sunbathing during gentler morning hours before 10 am. Younger adults can do it around noon, but not for too long.

Other medical professionals also warn that sunbathing carries health risks if not done prudently, including potential heatstroke, sunburns and complications for those with conditions such as cancer, autoimmune diseases or photosensitivity.