Saffron Walden police station has been closed to staff and the public after a number of officers were affected by chemical fumes.
A gas believed to be ammonia-based was detected at about 6pm on Saturday (October 10) when four officers reported feeling unwell.
Fire crews and paramedics were called and the fumes were found to be escaping from the former custody area.
The four officers were checked over but did not require medical treatment.
Chief Inspector Richard Melton, the Braintree and Uttlesford district commander, told Radio Essex: “Unfortunately this is the third time in a year that we have had to close Saffron Walden police station. First we had problems with wasps, then rats and now chemical fumes. The station will remain closed to staff and public while investigations continue.
“However, we can maintain the same level of policing as officers can be based elsewhere but can stay mobile and continue patrols by using our mobile data systems.