The reading in the downtown district of Central was 231.8 micrograms per cubic meter at 3 p.m. local time, data from the Environmental Protection Department show. The gas can cause significant inflammation of the airways once concentration levels exceed 200 micrograms, according to the Geneva-based World Health Organization.
Hong Kong is seeking to replace old diesel vehicles as aging buses and trucks have led to a worsening in air quality since 2007. Sunny, drier weather with milder wind has contributed to the build-up of pollution in the past few days, said Simon Ng, head of transport and sustainability research at independent think-tank Civic Exchange.
“The exposure level is very dangerous,” said Lai Hak-kan, who does research on environmental health at The University of Hong Kong’s School of Public Health. “At these levels, I would avoid strenuous exercise around those areas and try to go to another location.”
The reading in Central has been above toxic levels since 8 a.m. local time, hitting 361.7 micrograms at 10 a.m. The odorless gas is formed from emissions from vehicles and power plants.
The pollutant has been linked to damaged lung function in children and asthmatic attacks in people, Lai said. The government this year pledged to HK$12 billion ($1.5 billion) in subsidies to replace old diesel vehicles.
Of the 133,704 diesel commercial vehicles in Hong Kong, 38 percent are at least 12 years old, according to the government.
Hong Kong has three roadside pollution monitoring stations in the busy areas of Central, Mong Kok and Causeway Bay. The Central monitor is sandwiched between the Asian headquarters of JPMorgan Chase & Co. and a Tiffany & Co. outlet.
The reading at 10 a.m. in Causeway Bay, a popular district for shopping and dining, was 340.7 micrograms per cubic meter. The station recorded a 207.9 reading at 3 p.m.
“People are getting numb to the numbers,” said Ng. “It’s become a part of our lives, it’s happening every day, and it’s unfortunate because people are being exposed to toxins that have been proved carcinogenic.”