The biggest threat to the Taj is the unregulated number of visitors, which on some days is over 50,000. Constant treading wears down the marble floors. Presence of visitors increases humidity inside the building and the grease from their palms causes deposition of grime on the walls. The greater threat is to the red sandstone used in the Mehman Khana and the mosque on either side of the Taj. Unlike marble which is hard, red sandstone is porous. The weathering of the Mehman Khana is very obvious.