Notable events
| Start year | End year | Sector | Information | Impact |
| 1970 | 1985 | Iron and Steel | Decline in the energy intensive iron and steel industry and other heavy industries. | Decrease in emissions |
| 1986 | 2018 | Residential Combustion | Decline in the use of solid fuels (use of coal for domestic combustion has been restricted in the UK by the Clean Air Acts) in favour of gas and electricity. | Decrease in emissions |
| 1990 | 2018 | Stationary Combustion | Deregulation in the use of gas for electricity production. Â Increased use of Combined Cycle Gas Turbine (CCGT) stations and other gas fired plant rather than coal for electricity generation. | Decrease in emissions |
| 1992 | 2018 | Road Transport | Stricter Euro I - VI Â emission regulations come in on trucks and buses offsets increasing vehicle km. Â Impact takes time to have an impact as only new vehicles need to meet standards. | Decrease in emissions |
| 1993 | 1993 | Agriculture | Field burning of agricultural waste stopped after 1993. | Decrease in emissions |
| 2020 | 2020 | Road transport, aviation and railways | Because of measures in place during the COVID-19 pandemic, CO emissions from road transport, aviation and railways in 2020 were 33% and 67% lower than they had been in 2019 respectively. | Decrease in emissions |
| 2021 | 2021 | Road transport and aviation | As a result of easing COVID-19 restrictions, CO emissions from road transport and aviation in 2021 were 3.5% and 161% higher than they had been in 2020 respectively | Increase in emissions |
Carbon monoxide (CO) arises from incomplete fuel-combustion and is of concern mainly because of its effect on human health and its role in tropospheric ozone formation. It leads to a decreased uptake of oxygen by the lungs and can lead to a range of symptoms as the concentration increases. Road transport, in particular vehicles with petrol engines, was the largest source of carbon monoxide emissions. However, emissions from road transport have decreased by 97% between 1990 and 2021 as a result of tightened emission standards, and the importance of the road transport sector has declined as a result – from 64% of CO emissions in 1990 to just 11% in 2021. Since 2013, the most significant source of CO in the UK has been residential sector combustion: this source contributed 12% of emissions in 1990, but emissions have not reduced as fast as for other sources and so the sector now is responsible for nearly one-fifth of UK emissions. Emissions from industrial sources have, like those from road transport, been reduced significantly since 1990 due to the decline in the use of solid fuels in favour of gas and electricity, as well as a decline in the production of steel and non-ferrous metals. Over the period 1990-2020, UK emissions have decreased by 83%.