Pollution from the burning of Indonesia's rain forests appears to be claiming another casualty: physical fitness in Singapore.

Poor air quality is keeping people indoors, denting demand for outdoor exercise in the usually balmy city-state.

"We're an outdoor fitness company, with up to eight sessions a day outside," Lisa Clayton, a director of OzFit/UFit Bootcamps, said Wednesday. "We have had to cancel pretty much most of them over the last two weeks."

She said her company usually catered to as many as 100 people a day training outside, but has instead hosted bootcamps at indoor locations, such as condominium function rooms, for about 60 people a day.

Read More Haze engulfs Singapore as forest fires rage in Indonesia



What's colloquially called the haze, but is actually air pollution, is an annual event in Singapore as Indonesians deliberately set forest fires to clear land, but this year, it's lasted longer than usual because El Nino weather conditions mean there's less rain.

The Pollutant Standards Index, a global gauge of air quality, ranged between 97 and 111 in Singapore early on Monday, after several days of less-gritty levels under 100, according to the National Environment Agency (NEA).

A level between 100 and 200 indicates unhealthy air quality, while levels above 300 are considered hazardous. The gauge climbed over 300 in late September but hasn't yet topped the 400-plus mark hit in 2013.

Indonesia finally accepted Singapore's offer of personnel and equipment to help fight the forest fires on Wednesday after weeks of waffling over the decision.