due to moisture exhaled by tourists, which raised its humidity to such levels so that salt from the stone was crystallizing

due to moisture that tourists had exhaled, thereby raising its humidity to such levels that salt from the stone would crystallize

because tourists were exhaling moisture, which had raised the humidity within them to levels such that salt from the stone would crystallize

because of moisture that was exhaled by tourists raising the humidity within them to levels so high as to make the salt from the stone crystallize

because moisture exhaled by tourists had raised the humidity within them to such levels that salt from the stone was crystallizing