The fact that you are working in dusty environment is one of the prime reason for your persistant cough
. Therefore the Dr has given you nasal sprays which will reduce the allergic nose. If the cough is persisting with thick phlegm then its better that you receive some antibiotics for a week to reduce the infection if any.
I hope you found my response to be helpful and informative. If you have any additional concerns I would be happy to address them.