Passengers on Sydney's trains are breathing in more than double the normal levels of carbon dioxide found indoors, a report from an environmental monitoring group has found.
The testing, undertaken by environmental monitoring devices company OnSolution on the morning of February 6, found carbon dioxide levels in carriages reached above 2000 parts-per-million on some peak-hour train services in the city circle.
An OnSolution researcher jumped on board an air-conditioned train from Central to Town Hall at 7.52am and recorded a carbon dioxide level of 2846ppm.
On another air-conditioned journey from Town Hall to Wynyard at 7.57am, the carbon dioxide level reached 2666ppm.
The carbon dioxide levels decreased to 1101ppm outside the peak hour on an air-conditioned train at 9.52am, according to the research.
However, University of Sydney indoor environmental quality lab director Richard de Dear says while these numbers are above the limits, they're not alarming.
"In a building, the normal upper limit of carbon dioxide concentration would be 1000ppm ... it's generally accepted that the goal is to keep levels below that," Prof de Dear told AAP on Wednesday.
"It's nothing alarming ... for short exposures. It's not dangerous."
Prof de Dear said while carbon dioxide levels increase when more people are crammed into a space, increased ventilation can help bring it back down.
Transport for NSW has been contacted for comment.