Tile installation is commonly done directly onto a subfloor, usually made of plywood. Then, a thick-bed method of 1-1/4” mortar is applied to the subfloor ready for the tiles.

This is generally a costlier way of setting tiles because of labor costs for a very skilled installer taking careful time to do it correctly.
The less expensive cement board (also called backerboard) method can be installed over the subfloor. Cement board is Portland cement, commonly a 1/2” thick. The seams are usually taped with fiberglass mesh tape, presenting a smooth and level surface ready for mortar and tiles.
Modified latex-Portland cement mortar or thinset mortar can then be used to adhere the tile to the cement board or the subfloor. Specify the relatively odorless thinset mortars. Like their conventional counterparts, they are primarily made of Portland cement but usually without the odorous additives which are known to emit VOCs while curing. They are generally considered inert once cured.
Specify formaldehyde-free plywood for the sub-floor, since formaldehyde exposure can be problematic to workers, installers and end-users. Specify more sustainable grout for filling the spaces between tiles. The more chemically laden conventional grouts contain many chemical additives, often including epoxies and silicones. VOCs can be emitted during curing. Grouts requiring damp-curing are generally considered the most inert. If toxicity of grout or mortar is a concern, specifying larger tiles requiring less grouting joints to fill helps reduce the concern. Cured grout joints can be sealed with water glass, a clear, odorless sodium-silicate solution.
Tiled surfaces are easily cleaned and maintained with mild soap. Vinegar or hydrogen peroxide are often effective when stains appear.
LEED Credits: Possibly EQ Credit 3 - Construction IAQ Management Plan and/or EQ Credit 4 - Low-Emitting Materials.