Sickness levels among staff at Avon Fire and Rescue have tripled and are now the second highest in the country.
A health and safety report to Avon Fire Authority revealed 1,738 days were lost to illness in the first half of 2017/18.
A further 3,381 were lost in the first half of the 2018/19 financial year, with mental illness cited as a main cause.
Performance manager Simon Flood said the figures were "unfortunate".
The latest figures reveal fire service staff have already taken nearly twice the number of sick days in the first half of this year as they did in the whole of the previous 2017/18 year, according to the Local Democracy Reporting Service.
Mr Flood told members of the fire authority committee on Friday: "We benchmark against most other fire services.
"Unfortunately, we're the second worst performing with the second highest level of sickness."
There are 50 fire and rescue services in the UK but only 27 agree to voluntarily share sickness data.
Mental health issues at Avon account for more than a quarter of the sick days so far this year, it was revealed.
Bath and North East Somerset councillor Mark Shelford asked assistant chief fire officers Simon Shilton and Rob Davis to consider employing a mental health nurse and a physiotherapist full-time.
Mr Davis said: "We know our staff may be exposed to traumatic incidents which have the potential to cause long-term psychological ill-health or which have an impact on emotional wellbeing.
"As a service we are committed to supporting staff and continue to look at ways in which we can do this."
Last year's figures showed half of all illness incidents were related to work and home life stresses and post-traumatic stress disorder.
One employee had been suffering workplace stress for 18 months. Each day lost to illness costs the service about £150 in salaries.
Injuries and staff illness cost Avon Fire and Rescue more than £283,000 last year.
Avon was rated "inadequate" in its fire regulation work in December.