Grout is a cementatious material used between tiles. It is porous and allows moisture and dirt to penetrate it. The build up of the dirt and moisture in the porous grout means in the worst circumstances it can turn black as well as harbour odours related to these areas.
A fresh or clean grout filling between the tiles can help brighten the whole room; however since the smaller tiles with more grout filling is often used in the bathroom and on kitchen walls, they become highly susceptible to moisture which in turn causes the growth of mildew.
If the moisture is not controlled in these areas or preventive measures are not taken against it, the mildew is soon going to spread in the entire area and before you know it your bathroom or any other tiled area is going to look discoloured and dingy.
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