BOSTON -- Business and homeowners using natural gas expect their gas companies to be doing everything they can to ensure their safety. But Team 5 Investigates found two of the three major gas companies in Massachusetts falling short on following regulations designed to protect them.

For decades, gas companies have been required to inspect the piping that comes into homes and connects to gas meters. They're supposed to check every three years for corrosion and every five years for leaks.

"Recently the gas companies in this state have just adopted programs to start complying," said Mark McDonald, president of the New England Gas Workers Association. "There are hundreds of thousands of homes that have not been inspected,"

Doug Buchan, an energy expert who worked in the White House and owned his own gas company, also said homeowners should be concerned. "They should be very well concerned because basically this is why we have all the explosions happening."

In the Hopkinton explosion that killed Iris and Violet Carey in 2002, the state cited N-Star for failing to do those inspections in the required time.

Keyspan, now National Grid, was also cited in connection with the blast that leveled a home in Lexington, Mass., in 2005. "They're putting the lives of all their customers at risk," said Buchan.

It wasn't until 2008 that the state Department of Public Utilities finally took steps to make sure the gas companies are following the law, giving them another three years to get these inside inspections done.

So far, both companies are almost half way there.

NSTAR admits it has 51,800 inspections to do to bring it into compliance with federal regulations. The state Department of Public Utilities says National Grid has 173,598.

Team 5 Investigates visited a home in Salem, Mass., and found a corroded pipe that had gone uninspected for five years. "It's in the advanced stage of corrosion. It definitely needs to be addressed," said McDonald.

Gas shut-off valves--or gate boxes--are another problem. Regulations require gas companies to make sure they're accessible. But at the scene of two house explosions in Weston and in Groton, the shutoff valves were not easily found.

When Team 5 spot checked a child care center in West Roxbury, Mass., the emergency valve was easy to locate, but not accessible. "It was filled to the top with debris, there was no way to get a gate key on that valve," said McDonald.

What's worse, said Ed Kelly, president of the Boston Firefighter's Union, are the thousands of shutoff valves that have been paved over. "I think decisions were made to address the corporate bottom line that is certainly detrimental to public safety and the safety of the firefighters who respond to those incidents."

Both gas companies told Team 5 they try to make valves accessible when streets are re-paved and they're meeting the state's requirements. The DPU claims all critical valves are properly maintained.

Officials at the DPU tell Team 5 that both companies are "on track" to get all of their required inside inspections done by August of 2011. They also deny being lax with their oversight.

If you're concerned your home hasn't been inspected, or if you have any questions about your gas service you should call your gas company.

National Grid customers should call 1-800-233-5325. NSTAR customers should call 1-800-592-2000 and a customer service representative will direct you to the proper department.