Answer:
The air was stale and hot
Step-by-step explanation:
At that time, there were no regulations about how the Factories should handle their waste/pollutants. So most of them just dumped them to the environment near the factory.
As a result, those who lives in slums near the factories have to experience u stale and hot air that is caused by the production. This caused many of them to develop some sort of respiratory illness.