In a previous post I touched on the subject of conservation in discussing why properties often have reduced light levels, or restricted opening hours. There is much more to the conservation of a property and it's contents than meets the eye.
Light is just one factor that needs protecting against, as well as reducing the exposure, one common method for historical properties (and not just the National Trust) is to use special UV protecting film on windows.
In some cases humidity is an important factor, as it can cause canvases to stretch or become misshapen, wood to crack or even promote the growth of mould.
Dust and dirt of obvious contenders, dirt brought can be brought in by visitors, although I'm not suggesting that they are moving refuse tips. Just the dirt and dust brought in on your shoes can cause damage - in my property every surface is dusted every day - not polished - that's only done once a year, just wiped with a clean, dry lint-free cloth.
The other menace is from visitors again... Sorry - but it's true, and this time it's about touching. A lot of damage can be done to delicate surfaces and textiles just by the lightest of touches. The grease on your fingers and hands can be transferred and cause discolouration. Sometimes it's unintentional - a coat brushing against curtain may seem negligible, but over a year hundreds of coats could brush against the same spot and the damage builds up. It has been suggested that 1 year open to the public causes the same amount of environmental damage to a house and its contents as 25 years of family life.
Properties do what they can to minimise the impact but they would certainly appreciate visitors doing their bit and resisting the urge to catch a quick feel of a 300 year old bedspread.