How good is the air circulation in trains/buses/tubes?

[Hifly130]

Obviously with the talk of air circulation very at the forefront of people’s minds I was interested to hear if anyone knows how often air is recirculated and what filtration is used in most public transport.

For instance in an Airbus the air in the cabin inflight is replenished every 3 minutes and HEPA ( medical theatre standard) filters are used which capture bacteria and viruses. Just wondering if there’s similar comparisons?