The darker the smoke, the more polluting it tends to be. Smoke darker than a specified shade of grey is officially classified as 'dark smoke'.
The Ringelmann chart is used to define dark smoke. The chart has 5 shades of grey with 0 being clear and 5 being black. Smoke is considered 'dark' if it is shade 2 or darker.
Chimney and boiler restrictions
You mustn't release dark smoke from your premises, including from:
- chimneys serving furnaces
- fixed boilers or industrial plants, whether they're attached to buildings or not
There are some exemptions if emissions won't damage health or cause a nuisance.