Air quality
If you are healthy and stay in a city with air pollution for a short period of time (less than a few weeks), the risk of health issues is low. However, if you have sensitive airways and use an inhaler for example, you could become unwell due to poor air quality.
Air pollution can have various causes: emissions from industry and traffic combined with weather conditions, such as dry weather with little wind, or, conversely, strong winds that can carry desert dust or particles from wildfires, leading to smog.
For the current status of air quality at your destination, you can check the Air Quality Index (AQI) here: World's Air Pollution: Real-time Air Quality Index.