Using wood in a bathroom is fine. If you dont already know this, take alook and you will see that the cabinets are usually finshed. However, if you have a window in your shower, it should be vinyl, NEVER wood.

This is not just my personal opinion, but one of professionals in the industry. Even if you coat it, it will eventually rot and need to be replace. Even wooden decks on the beach, which are ply wood, and TREATED, it stilll will eventually need replacement.

Dont use wood in the shower are of the bathroom window.

All other places in your bathroom use the appropriate materials.

Bathroom
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A smaller U.S. residential bathroom. A bathroom is a room that may have different functions depending on the cultural context. In the most literal sense, the word bathroom means "a room with a bath". Because the traditional bathtubs have partly made way for modern showers, including steam showers, the more general definition is "a room where one bathes". There can be just a shower, just a bathtub or both; and often both plumbing fixtures are combined in the bathtub. The room may also contain a sink, often called a "wash basin", "hand basin" or "lavatory", and often a toilet fixture.

In the United States, "bathroom" commonly means "a room containing a toilet". In other countries this is usually called the "toilet" or alternatively "water closet" (WC), lavatory or "loo". The word "bathroom" is also used in the U.S. for a public toilet (the more formal U.S. term being "restroom").

In the United States, bathrooms are generally categorized as a "full bathroom" (or "full bath"), containing four plumbing fixtures: bathtub, shower, toilet, and sink; "half (1/2) bath" (or "powder room") containing just a toilet and sink; and "3/4 bath" containing toilet, sink, and shower, although the terms vary from market to market. In some U.S. markets, a toilet, sink, and shower are considered a "full bath". This lack of a single, universal definition commonly results in discrepancies between advertised and actual number of baths in real estate listings. An additional complication is that there are currently two ways of notating the number of bathrooms in a dwelling. One method is to count a half bathroom as ".5" and then add this to the number of full bathrooms (e.g., "2.5" baths would mean 2 full baths and 1 half bath). The other, newer method is to put the number of full bathrooms to the left side of the decimal point and to put the number of half bathrooms to the right of the decimal point (e.g., "2.1" would mean 2 full baths and 1 half bath; "3.2" would mean 3 full baths and 2 half baths).