When you can see it.
The "fragrant harbor" from which Hong Kong takes its name is often shrouded in toxic smog.
Hong Kong's air pollution hit a record high on September 14, with the index rising to over 200 for the first time since air quality monitoring was introduced in 1995. Doctors advised asthma sufferers and those with heart disease to stay indoors.
"I don't see any reason why it will improve. The trend is really alarming," said Alexis Lau, acting director of the Center for Coastal and Atmosphere Research and a professor at the Hong Kong University of Science and Technology.
Experts lay the blame for Hong Kong's worsening air pollution on China, which the World Health Organization says has seven of the world's 10 most polluted cities.
Most of the pollution cloaking Hong Kong is generated by coal-fired power plants and smokestacks from China's industrial south, as well as traffic fumes from the city's own congested streets.
China faces a chronic energy shortage and making the switch from coal-fired plants to cleaner fuel is costly and disruptive.
It is the second consecutive summer that factories have faced brownouts or managed power cuts. Residents have been urged to turn off their electricity-guzzling air conditioners despite the sweltering heat.
Double-digit growth in individual car ownership in the neighboring province of Guangdong compounds the problem.
In Hong Kong, the number of days a year when visibility falls below 8 km (5 miles) has risen from around 50 in 1993 to over 160 last year. Nearby Shenzhen and Macau have seen similar increases, according to data provided by Lau. This week, pollution was worst near Hong Kong's airport, where planes took off and landed in an orange-tinged haze. One day in August, visibility in Hong Kong harbour fell to as low as 550 meters (600 yards) and smog contributed to collisions on the water involving eight vessels.
The problem is set to get worse this winter when prevailing northern winds sweep the sulphur dioxide and other toxins belched out of China's smokestacks toward Hong Kong, said Lau.
But not all of Hong Kong's residents are worried about the deterioration in air quality.
"I think the pollution is more or less the same. I can't say I think about it too much," said Wong Wai Ming, a delivery man who spends four to five hours outside each day.
Hong Kong has taken several steps to reduce pollution in the past decade, including conversion of taxis to liquefied petroleum gas (LPG). Air pollution readings are taken hourly and are a regular feature of weather forecasts.
"For roadside pollution we've seen a significant improvement but regional pollution seems to be deteriorating. There's more smog than before," said Raymond Leung, principal environmental protection officer.
Activists say the city's pro-business leaders are not doing enough. They say the government should do more about idling vehicles and introduce electronic road pricing to reduce congestion.
"I've heard of people saying that they will consider leaving Hong Kong because of the bad pollution," said Edwin Lau, assistant director of Friends of the Earth (HK).
"Tourism is a huge part of our economy and if the pollution deterred tourists it would be a big loss," he added.
Electricity provider CLP Holdings, which steadily reduced its emission of pollutants in the 1990s, reversed progress last year by burning 50 percent more coal than in 2002 and cutting its use of gas, a cleaner fuel.
Reserves in the South China Sea gas field on which it relied were overestimated,. forcing it to burn more coal to meet rising demand here and in China but CLP said it would maintain a balanced fuel mix of coal, gas and nuclear in the long run. Another problem in battling the smog is the thorny issue of cooperation with China. Priorities in the former British colony, where public concern over the environment has grown in recent years, differ from those over the border.
China's leaders are aware of the environmental price of breakneck growth but their main priority is to ensure a strong economy to help ease a labor glut.
To reduce regional air pollution, Hong Kong and the Guangdong provincial government have set a target to reduce emissions of sulphur dioxide in the Pearl River Delta by 40 percent by 2010.
"It's a big challenge - due to economic growth in the area there's much higher demand for electricity," said Leung.