and interior lining), overfill protection (to prevent overfills of the tank during tank filling operations), spill containment (to catch spills when filling), and leak detection for both the tank and piping. Many USTs were removed without replacement during the 10-year program. Many thousands of old underground tanks were replaced with newer tanks made of corrosion resistant materials (such as fiberglass, steel clad with a thick FRP shell, and well-coated steel with galvanic anodes) and others constructed as double walled tanks to form an interstice between two tank walls (a tank within a tank) which allowed for the detection of leaks from the inner or outer tank wall through monitoring of the interstice using vacuum, pressure or a liquid sensor probe. Piping was replaced during the same period with much of the new piping being double-wall construction and made of fiberglass or plastic materials. Tank monitoring systems capable of detecting small leaks (must be capable of detecting a 0.1 gallons-per-hour with a probability of detection of 95% or greater and a probability of false alarm of 5% or less) were installed and other methods were adopted to alert the tank operator of leaks and potential leaks. U.S. regulations required that UST cathodic protection systems be tested by a cathodic protection expert (minimum every three years) and that systems be monitored to ensure continued compliant operation. Some industrial owners, who previously stored fuel in underground tanks, switched to above-ground tanks to avoid environmental regulations that require monitoring of fuel storage. Many states, however, do not permit above-ground storage of motor fuel for resale to the public. The EPA Underground Storage Tank Program is considered to have been very successful. The national inventory of underground tanks has been reduced by more than half, and most of the rest have been replaced or upgraded to much safer standards. Of the approximately one million underground storage tanks sites in the United States as of 2008, most of which handled some type of fuel, an estimated 500,000 have had leaks. , there were approximately 600,000 active USTs at 223,000 sites subject to federal regulation. In 2012, EPA published how to screen buildings vulnerable to petroleum vapor intrusion, and in June 2015, U.S. EPA finally released its "Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air" and "Technical Guide For Addressing Petroleum Vapor Intrusion At Leaking Underground Storage Tank