How often should we replace old Helmet with new?

[Vishnu Janardhanan S]

Dear All,

I know that I am not supposed to use a helmet after a crash.

Other than that

How often should we change the helmet?

What are the signs that a helmet needs replacement?

Another question i had regarding helmets is

I commute to work in cycles so i use it every day. 

I sweat a lot and i end up washing the pads and the strap too often, even then it does not feel clean.

Any solutions?

Thank you,

Regards

Vishnu