Sunday 14 May 2017 18.18 BST
They take their inspiration from the well-known signs linking people from the past with the buildings they once inhabited, but the symbols now appearing across London are to highlight a different connection.
In the past week, grey plaques – direct copies of the English Heritage blue plaques identifying the homes of the dead and famous – have been put up on buildings across the capital to identify streets and houses in areas where air pollution threatens public health.
Each plaque carries the phrase London’s Choking to point out areas where levels of NO2 – predominantly from diesel traffic – regularly reach levels that are harmful to human health.
Joe Dennett and Rob Donaldson came up with the idea to try to raise awareness of the invisible threat from air pollution to tens of thousands of Londoners.
“It is an issue which Londoners are becoming more aware of, and which we have become increasingly concerned about. But we realised a lot of people were not aware of where pollution levels are high because you cannot see the pollution, it’s quite nebulous.
“We wanted to try and create awareness and anger about it at grass roots and to come up with something that would identify the air pollution.
“The English Heritage blue plaques highlight the invisible past of a building and this is trying to highlight the invisible danger of the pollution in the areas where the grey plaques are being put up.”
The first plaque – with a skull and crossbones at its base – appeared on Brixton Road, which by 4 January this year had breached annual legal limits for NO2 pollution. Further plaques in Putney High Street, Farringdon Street and Oxford Street have also been erected.