Zero Emission Zones (ZEZs) such as Oxford's should be reworked with general access restrictions that address all the harms done by motor vehicles, not just tailpipe emissions.
The goals of Oxford's ZEZ are to "improve the air quality in Oxford and reduce traffic" by "encouraging people to switch to low and zero emission vehicles" and "making other positive changes to their travel behaviour".
But the ZEZ charging scheme only considers tailpipe emissions. It fails to address particulate air pollution, which is generated by electric vehicles at similar levels to petrol or diesel ones.
Although lightweight EVs emit an estimated 11-13% less PM2.5 than ICEV equivalents, heavier weight EVs emit an estimated 3-8% more PM2.5 than ICEVs. In the absence of targeted policies to reduce non-exhaust emissions, consumer preferences for greater autonomy and larger vehicle size could therefore drive an increase in PM2.5 emissions in future years with the uptake of heavier EVs. ("Non-exhaust Particulate Emissions from Road Transport", OECD 2020)
(It's worth noting here that, while the current legal limit for PM2.5 in the UK is 25μg/m³, PM2.5 levels under 10μg/m³ have significant adverse effects and the World Health Organisation recommendation is that they be kept under 5μg/m³.)
The ZEZ charging scheme also fails to address road danger, which is actually worse with electric vehicles because they are larger and heavier for the same capacity, and accelerate faster. And it fails to address other harms to the public realm, notably the space taken up by vehicles (moving and parking) and noise pollution.
There is a clear public benefit in encouraging people to switch to lighter and smaller vehicles, not just to lower emission ones. A large electric SUV may create as much air pollution as a small petrol car and certainly does more to make the city centre dangerous for people walking or cycling; this should be reflected in the charge it pays to enter the city centre.
The ideal is a Pigouvian tax which captures all the negative externalities created by motor vehicles. This would involve charging based on size and weight as well as on emissions: weight would serve as a proxy for particulate emissions and size as a proxy for road danger. (In practice these may be so closely correlated that using a single size-weight scale would make sense.) And petrol mopeds and motorcycles could be charged more because of their over-size contribution to noise pollution.