The ozone count in central Kolkata was 60 per cent more than the safe levels, raising concerns about public health and the threat it poses to the iconic Victoria Memorial.

The eight-hour average was 160 mg/cubic metre in the Victoria Memorial area on Sunday, as per the West Bengal Pollution Control Board (WBPCB).

The eight-hour average of ozone should not exceed 100 mg/cubic metre, as per the World Health Organisation (WHO).

The high ozone level was a result of typical wintry conditions and accumulated vehicular emission particles in the lower atmosphere, a WBPCB official said.

Once the foggy conditions improve, the ozone count will go down, he said.

Environmentalist Somendranath Ghosh said proper monitoring is needed to find the reasons behind the spike in ozone levels in the eco-sensitive zone of the city, often referred to as the 'Lungs of Kolkata' because of the acres of grassland.

It is alarming the ozone count even eclipsed the PM 2.5 level of 144 on Sunday noon, he said.

"So many old diesel vehicles are plying. Due to this pollution, the white marbles of Victoria Memorial may turn yellow and the plasters will go off in many chamber walls in the future," he cautioned.

Ground-level ozone is a colourless and highly irritating gas that forms just above the earth's surface, Ghosh said.

It is a secondary pollutant as it is produced when two primary pollutants -- nitrogen oxides (NOx) and volatile organic compounds (VOCs) -- react in sunlight and stagnant air, he said.

Ozone can cause the muscles in the airways to constrict, trapping air in the alveoli, leading to wheezing and shortness of breath.

"Depending on the level of exposure, ozone can cause coughing and sore or scratchy throat," the WBPCB official said.