Natural gas seeping from pipes caused an explosion at a San Diego hotel construction site that injured a firefighter and 13 construction workers. Three of those injured required weeks of hospital care for critical burns, according to the Union-Tribune. The hotel’s fourth through seventh floors were destroyed in the explosion.

Cal-OSHA assigned most of the blame for the blast to Sherwood Mechanical, Inc., a plumbing and heating contractor. Sherwood faces fines of $67,500 for four serious violations:

• The company’s Code of Safe Practices didn’t address methods for testing concentrations of natural gas, procedures for purging gas line or methods to control operation of gas.
• Employees weren’t instructed on methods for testing natural gas concentrations, procedures for purging gas or methods to control access to valves and pipes.
• Piping being purged of air wasn’t vented outside from an enclosed space, and the vent wasn’t closed after air was purged.
• The company allowed natural gas to accumulate in a room to dangerous levels and failed to test the concentration of gas in the room.

A report says workers trying to flush air out of pipes feeding a hotel boiler inadvertently allowed natural gas to leak into a fifth-floor mechanical room.

After the gas built up in the room, any number of electrical installations could have been the ignition source, including light switches and lighting itself.