Your question is way to general and you have not provided enough information. A dry cough can be caused from many things such as a common cold all the way to something more serious such as an upper respiratory tract infection. If you want to re-submit a question with more details we may be able to help you out better or I always suggest when something like a dry cough is lasting for this long (one month in your case), that you go see your family doctor in person to go get it checked out.
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