Most vehicles already have a tint applied to the glass that keeps out ultraviolet rays.  Aftermarket tint can be added to make windows darker.  However, this sort of tint is often below state or country regulations.  It should be noted that these laws may change depending on type of window.  A back window may have a legal amount of tint on it, but the same amount could be illegal on a front window.

Some medical conditions require a limited exposure to sunlight, such as sunlight allergies or photosensitivity.  Proper documentation showing the medical necessity (prescription or letter from a medical professional), the amount of light to be limited, prescription expiration date or permit duration, and the particular vehicle for which the medically necessary tint applies should be kept in the vehicle.

Tint laws may vary from state to state.  It is the responsibility of the driver to be aware of the various tint laws when travelling in other states.

Visit the Tint Laws page for state and international tint law information.