The department says staffing at 49 of its 194 companies will be reduced from five to four starting Wednesday.
Department officials said Tuesday an increased medical absentee rate of 7.53 percent prompted the reduction. They say more leaves cost the city more in overtime.
The department says the reductions won't affect its response time of 4 minutes and 1 second.
But the fire union says the cuts will put residents and firefighters at greater risk. It says studies have shown that such reductions double the time it takes to get water on the fire.
The department plans to review the leave rate again on Jan. 1.
This is the third decrease in seven years blamed on firefighter absentee rates.