You can smell it and often identify what it it, they knew.

The Romans had court cases relating to air quality and London was suffering in the 1200s. In the 1300s London started legislating to try and control the pollution.

https://www.smithsonianmag.com/science-nature/air-pollution-...

https://www.londonair.org.uk/londonair/guide/londonhistory.a...