The UK high court has ordered the government to publish its air quality plan by May 9. The move comes after the government was criticised for its last-minute attemps to delay the plan until after the general election on June 8, due to “purdah” rules which limit announcements during the election period.

The court had previously ordered the government to urgently revise its strategy for dealing with air pollution, after legally-binding EU limits of toxic pollutants were breached in towns and cities across Britain.

In November 2016, the court ruled an earlier version of the plan was illegally poor, and it had failed to bring levels of nitrogen dioxide (known as NO₂) down to legal limits in “the shortest possible time”.

The latest ruling means the revised plan must be published by May 9, with the final policy published on the original deadline of July 31. The government now has the option of going to the court of appeal, and by the time any appeal is heard it would likely be beyond the election anyway.

Air pollution is a major issue

The problem is too urgent for such legal holdups. Each year there are around 40,000 deaths due to outdoor air pollution, and the UK has the second highest mortality rate in Europe associated with NO₂ emissions. In busy city centres, the mortality rates are higher.

So what can be done about it? In cities, emissions from road transport are the biggest source of harmful nitrogen oxides (NOx) and fine particles known as particulate matter (PM). To tackle the problem a range of new measures are needed, some of which would be unpopular with many motorists (voters).

Proposals would likely include higher rates of tax for the dirtiest cars and trucks, as well as clean air zones such as the Ultra Low Emissions Zone (ULEZ) in London. The ULEZ, which is due to come into force in 2019, will involve a £12.50 charge (in addition to the current congestion charge of £11.50) for the most polluting passenger vehicles.

Of course, these higher charges will devalue the very diesel vehicles which had been promoted by previous governments. To help, the government has been considering a scrappage scheme (based on a similar scheme in France), where diesel owners would be offered incentives to trade their vehicles for less polluting models.

Diesel vehicles were promoted throughout Europe in the 1990s as they are more efficient than petrol cars in terms of CO 2 emissions, and thus emit fewer greenhouse gases. However, diesel vehicles emit higher levels of nitrogen oxides and particulate matter, the emissions that affect human health.

It’s true that diesel vehicles are now much cleaner than they were in 1992, when the first round of European emissions legislation (EURO I) came into force. The regulated level of harmful particulate emissions has reduced by about 97% in that time, with similar trends for NOx.

Diesel engines still emit more NOx than petrol engines, and there are increasing numbers of diesels on British roads. Car makers have already implemented many of the most obvious and cost-effective measures such as diesel particulate filters. This means that that further reductions of diesel emissions are likely to be more gradual.

This year to date more than 98% of new vehicles registered in the UK still have an internal combustion engine – running on diesel or petrol fuels. While these newer vehicles are less polluting than older models, there are no safe levels for toxic pollutants such as airborne particulate matter.

Fully electric vehicles will take time to catch on, just as the required charging infrastructure will take time to mature. Part of the problem is that drivers are anxious about the range of electric vehicles, which is typically around 100 miles. They needn’t be, in London 66% of car journeys are under 5km. Encouraging the uptake of electric vehicles in cities needs a widely available public network of charging points.

But, while the technology and infrastructure is improving, we don’t have time to wait for electric cars. The air quality problem needs immediate solutions. To clean up the air the government’s plan will need to include higher taxes on the dirtiest of vehicles, and low-emissions zones in most cities.

This article was updated on April 28 to include the latest high court ruling.