Singapore

CO2 levels around 1300 on the subway, despite it being quite crowded.

Not too bad!

The medium-high CO2 and low PM2.5 in a crowded space suggest that both ventilation and filtration are being used.

Indeed, both filtration and ventilation are used at our underground subway stations. 

Here's an article on a local newspaper with relevant details shared by our local transport regulator:

Indeed, both filtration and ventilation are used at our underground subway stations. 

Here's an article on a local newspaper with relevant details shared by our local transport regulator:
https://www.nus.edu.sg/newshub/news/2022/2022-12/2022-12-18/AIR-st-18dec-pA16.pdf