From the options given above smog and vehicle exhaust are outdoor pollutants. Oxygen is not a pollutant at all. The only option left is natural gas. When we cook food or use heaters we use natural gas so the burning of natural gas pollutes the indoor environment that is why we can say that natural gas is indoor pollutants.
Critical Access Hospitals (CAH) are small facilities that give limited outpatient and inpatient hospital services to people in rural areas that receive cost-based reimbursement
