BEIJING: A Chinese entrepreneur is selling fresh air in soft drink cans, similar to bottled drinking water, as north China is once again choking in toxic smog.

The concentration of airborne PM 2.5 particulates - the smallest and most deadly - went off the chart in the early hours of Tuesday morning for the second time this month, according to the pollution gauge at the American embassy in Beijing.

The air quality index, designed by the US Environmental Protection Agency, cannot cope with levels beyond 500, which is 20 times the World Health Organisation air quality standard.

The embassy gauge had been hovering in the merely ''hazardous'' 300-500 range since Friday.