The U.S. Department of Transportation has issued an advisory notice warning that composite cylinders used in self-contained breathing apparatus should have a maximum of 15 years. Once they are 15 years old, the cylinder must be discarded because they could have reduced strength without any visual indication of damage. DOT's advisory comes in response to numerous letters from fire departments expressing concern about the cost of replacing the cylinders.

The composite cylinders involved consist of a metal liner surrounded by fiber-reinforced plastic, usually fiberglass or Kevlar. The majority of the strength of such cylinder comes from the thousands of fiber filaments in the outer shell, which grow susceptible to cracks and fractures over the course of years.

Once the filaments develop a crack, the crack will continue to grow until the filaments break. Unfortunately, filaments cannot be manufactured without some cracks. More cracks are caused in the inevitable wear and tear of service. With each crack, more of the load is transferred to other filaments which consequently crack faster. Eventually, the cylinder could become so weak it ruptures.

In addition to the problem of cracking fibers, the fiberglass shell is also susceptible to a gradual loss of strength over years of containing high pressures, leading to the chance it could simply burst from normal service air pressure.

DOT warned that air tank failure during use could be extremely dangerous, noting that a fire fighter in New York state was killed when an air tank burst last year. The department established the 15 year limit because there is no test available to determine whether or not a cylinder has become so weak is should be removed from service.