Under rule 123 of the Highway Code, drivers must not leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road.
Areas outside schools have been identified as high risk sites where emissions pose particular concerns.
Hospitals, GP surgeries, care homes and other healthcare facilities are also identified as high risk locations.
Professor Isabel Oliver, chief medical officer for Wales, said air pollution was a serious public health issue that contributed to a range of conditions including heart and lung disease.
“Children, older people and those with existing health conditions are particularly vulnerable to its effects,” she said.
“Idling may seem like a small, everyday habit, but it adds up.
“Every time an engine is switched off rather than left running, we reduce the amount of harmful pollution in the air we all breathe, particularly around schools, hospitals and other places where vulnerable people spend time.”
This new measure comes into force following a vote at the Senedd in March.
MSs passed the new rules, with 37 politicians voting for and 12 against. There were no abstentions.