Shell’s subsidiary, Shell Oil Products has paid nearly $3 million in penalties at its California refinery following an equipment failure that sent nearly 1,000 tons of excess carbon monoxide into the air.

The pollution escaped the refinery in Martinez back in March after three boilers that simultaneously produce steam and control the plant’s carbon monoxide output were shut down.

Karen M. Schkolnick, a spokeswoman for the Bay Area Air Quality Management District said “It was a series of either bad judgments or mechanical failures and it led to this acute situation”.