The enemy: Solar radiation...alias: Sunshine...alias: ultra-violet light.
Direct sunlight is very, very hard on any mat, but especially non-conservation matboard. Fading of mat color can occur extremely quickly in late spring, summer and early fall when sun is high, and is still a major concern at any time of year. The core of mat will go pre-maturely brown with high exposure. The artwork itself is also at risk, especially lower quality prints. Photos printed by high volume retailers can fade in a couple of days of strong sun. Photos from higher quality retail photo finishing shops are often of much higher quality, but will still fade with enough exposure to direct sunlight.
The cure is easy. Never allow framed art of any type to be exposed to direct sun. Period. Unfortunately florescent lighting has some of same harmful qualities of sunlight, though to a lessor extent. If artwork will be exposed to high volumes of this type of lighting, consider purchasing conservation quality glass. It is available at most glass shops and in custom framing retail stores. Prepare to be shocked, as price is very high. Regular and non-glare glass afford some UV protection, but not much. Acrylics, (plastic, plexiglass, etc.), have inherent properties that reduce some ultraviolet light but cannot be considered conservation quality.
The enemy: High relative humidity or excess moisture.
Definition of Relative Humidity: The amount of water vapor in air relative to amount of water vapor air is capable of holding at a given temperature. If temperature goes down, relative humidity will go up. If temperature goes down enough, air cannot hold moisture and condensation occurs.
Excess moisture in air will penetrate framing package, causing condensation. The problem is most apparent when a slight bit of cooling allows water vapor to condense on glass. Any adjacent surface will suffer water damage. If a mat is present at least artwork is protected from direct damage, which is an excellent reason to use mats. Most artwork will tend to become wavy.
Under normal conditions this should not be a big problem. If temperature inside is warmer than outside, relative humidity should remain below danger point. Unheated areas can sometimes produce relative humidities at or near 100%, and certainly bathrooms and kitchens can be a problem. Shipping artwork, or allowing it to be transported in vehicles during periods of high heat and humidity can be deadly if care is not taken. Moisture absorbers can be packed with artwork to help protect it. Artwork in kitchens and bathrooms can be sealed to some extent by caulking edges, but this is only a partial help. Never hang valuable artwork in a high humidity area. Ever.