Ministers cited election purdah rules on policy announcements.
Environment Secretary Andrea Leadsom told MPs that it was “not appropriate” to publish the plan during the pre-election period.
University researchers examined the effect of nitrogen dioxide (NO2) on life satisfaction.
Mayor Sadiq Khan has described the capital’s air as “lethal”.
The link will run from the A2 east of Gravesend – near the M2 – and beneath the river to join the M25 in Essex between junctions 29 and 30.
Greenpeace says throwaway drinks bottles are the most common type of plastic packaging found washed up on shorelines.
The PM says drivers who were encouraged to switch away from petrol under Tony Blair’s government will be taken into account.
Hundreds of doctors, nurses and other health workers are calling on the Prime Minister to take action to get polluting diesel vehicles off the UK's roads as soon as possible.
The UK faces being taken to court over air pollution after the European Commission sent a "final warning" for repeated breaches of legal limits in parts of the country.