Plans to reduce high-polluting vehicles travelling around UK cities are set to be delayed due to the coronavirus crisis.
Councils in Birmingham, Oxford and Leeds have written to the government asking permission to postpone their plans to reduce city centre traffic.
At the same time, levels of nitrogen dioxide (NO2) have fallen since people were told to work from home last week.
The government said it was “sympathetic” about Covid-19’s impact.
Birmingham was among five cities the government called on to establish a clean air zone along with Leeds, Southampton, Nottingham and Derby.
A spokesman for the Department of Environment, Food and Rural Affairs (Defra) said: “We are sympathetic to the challenges local authorities are facing and we are working with them to mitigate the impact this may have on other priorities.
“We have received Birmingham’s request to delay the implementation of its Clean Air Zone, and will respond as soon as possible.”
In some city centres average daily nitrogen dioxide emissions have fallen compared with the same eight-day period last year, analysis of Defra data suggested.
Nitrogen dioxide, released from car exhausts, is a serious air pollutant and also indirectly contributes to the warming of the planet.
On 16 March, Prime Minister Boris Johnson urged people to work from home and avoid non-essential travel, going on to close schools and instruct people to stay home except for very limited reasons.
Birmingham’s Clean Air Zone (CAZ) was due in July, but the council has asked for government permission to postpone it until “at least” the end of the year.
The launch of a Zero Emission Zone (ZEZ) in Oxford was due in December, and motorists not driving zero emission vehicles in the city centre were to face charges from February next year
Oxford City Council and Oxfordshire County Council have now closed a consultation into the scheme.
It will resume later in the year with a “view to implement the scheme in the summer of 2021”, they said.
Councillor Tom Hayes, cabinet member for Zero Carbon Oxford, said: “We are all living through an unprecedented crisis. We have to get our priorities right at this time.”
Leeds City Council was due to launch a Clean Air Charging Zone in September.
Council leader Judith Blake said it would “not be appropriate” to work on introducing the measures yet.
“We are asking for more time, it’s not appropriate at the moment to move forward with the current timetable,” she said.
Southampton has been operating a non-charging clear air zone since 2019, while Derby is still consulting on its plans.
Council leaders in Nottingham, however, have opted to introduce other measures to curb air pollution.
The BBC has approached other cities planning clean air zones for comment.
Reporting team: BBC England data team, Shared Data Unit, Local Democracy Reporting Service