The San Bruno gas explosion has brought up a number of questions including the age of the steel pipes used in San Diego's gas infrastructure.

Critics have also pointed to regulation and said gas companies are not watched closely enough.

"We need more regulation. We need more awareness. I think we need to have local governments look at this to keep people safe around pipelines," said Carl Weimer with a non-profit advocacy group called the Pipeline Safety Trust.

Some experts have said steel pipes that carry natural gas have a shelf life of approximately 50 years.

Officials with SDG&E confirmed that many of the gas lines in San Diego County are around that age but refute the notion that the pipelines are breaking down because of age.

"The majority of our pipelines are from the 50s and 60s but it's a misnomer to equate age with condition," said Bret Lane, vice president of field services for SDG&E and Southern California Gas. "If you keep the pipeline maintained it doesn't really deteriorate."

The large gas line that exploded in San Bruno was what they call a "transmission line" and those pipelines are generally much larger than "distribution lines" which are used to carry gas to individual homes.

According to recent reports, between 2002 and 2009, San Diego County has had zero incidents with transmission lines and five incidents regarding distribution lines.

There was one injury reported as a result of those incidences.

Gas companies in California are regulated on the federal level but also on the state level by the California Public Utilities Commission.

"PG&E conducts required leak surveys annually and patrols the transmission lines twice a year," said CPUC spokesperson, Terrie Prosper in an e-mail. "The CPUC can assess penalties to utilities for non-compliance."

Despite the two regulatory bodies, the Pipeline Safety Trust says gas companies are not required to check the vast majority of the more than 300 thousand miles of transmission lines found in the state.

The deadly explosion in the northern California suburb has rattled many across the nation but Weimer says an accident like that will continue to be rare.

"The chances of something like happening are really, really small," Weimer said. "It's just when there are breakdowns, the consequences are really severe."

More information about gas pipelines can be found on the Pipeline Safety Trust, click here.