Smoking (active or passive) is another thing that effects the throat. 
Although a lot of singers like the quality of voice that smoking gives them, evidence shows that it can cause a harsh voice, lower your pitch and the top end of your range suffers. Not only that, it effects your breathing and voice quality. It can also lead to mouth and throat cancer. 

Smoking also contributes to chronic laryngitis when the vocal cords are thickened and look red instead of their normal white. If you smoke, your throat may become dry and sore, causing you to keep clearing your throat and thus resulting in further irritation.

Central heating can also dry your throat up.
Certain medication will have the same effect.
And lastly, tension in the throat.

GIVE YOUR VOICE A REST
From time to time, give your voice a rest and keep quiet! 

OTHER WARNING SIGNS
There are many factors that can affect your voice, so get used to listening to your body and catching the signs early: