The current standard filtration systems require filtering both air and water. For air, you’ll want an NBC filter system, which both domestic and international producers make. NBC stands for Nuclear, Biological, and Chemical, which filters out all three. If you’re only building your shelter for natural disasters, chemical attacks, and biological attacks, you may opt for the less expensive HEPA filter system.  Water filtration requires the same level of care. You’ll likely want to have a large water storage capacity to survive short-term disasters. For long-term living, you’ll want to have a water filtration system.