The government has long blamed high pollution levels in Hong Kong largely on emissions from factories and power plants in Guangdong province. Clear skies of late have been put down to effective policies on both sides of the border. This newspaper's study of data from the Environmental Protection Department's monitoring stations certainly shows an improvement in air quality at the rooftop level over the past four years. Alarmingly, though, what we breathe on our streets has become dangerously unhealthy.

We found that during the first half of this year, the air pollution level at roadside stations in Central, Causeway Bay and Mong Kok was above the "very high" 100 mark for 1,066 hours, or the equivalent of more than 44 days. The figure was more than six times worse than in 2005. Predictably, authorities have responded by pointing to Guangdong. They say regionally produced ozone is mixing with pollutants which descend to street level, causing the high readings.

The explanation is at odds with what scientists we have spoken to contend. They say that purely and simply, the problem is homegrown. Vehicle exhaust fumes are mostly at fault.

Scientific study of pollution is not exact. A multitude of often complex factors can cause poor air quality. Findings are open to interpretation. But there can be no quibbling with the data; in this case, it shows that we are increasingly exposed to unhealthy air when we take to our busiest streets.

It is wrong for officials to shrug their shoulders and say there is little they can do because the source of the problem is out of their reach. Their lack of urgency in tackling pollution from vehicles reveals an ignorance of data that their own environmental department has collected. Policies have been implemented, but they are clearly not sufficient. Options long available to the government, ranging from action on idling vehicle engines to electronic road pricing remain untried. The approach belies the seriousness of the problem.

The government is not ignoring the problem. Taxis and many buses now run on clean fuel. Tax breaks are on offer for buyers of environmentally friendly cars. Nonetheless, the poor air quality readings show that a tougher stand is necessary.

A sound start would be to adopt higher World Health Organisation standards than those proposed by a review of our air quality objectives, and to put in place a timetable for their implementation. The government has suggested that the least-stringent control level be adopted for the key pollutants sulphur dioxide, ozone and fine air particles. Legislation banning idling vehicles with running engines has to be promptly put before lawmakers. Next, concerted effort must be made to rid our roads of the tens of thousands of old buses and trucks with polluting diesel engines. Road pricing and pedestrian streets must be among measures given serious consideration.

The health risks of high pollution levels are well proven. Hong Kong cannot afford to take the least intrusive steps. The problem is a collective one, but the government has to take the lead. Tough action is needed and it has to be taken quickly.