The use of a potent pepper spray may be behind a drop in violence in city jails this year, the Department of Correction commissioner, Martin Horn, said yesterday. Violent attacks involving weapons reached the lowest levels in city jails in more than a decade as the use of force by guards, including subduing inmates with a new concentrated chemical spray, has increased, Mr. Horn said.

Slashings and stabbings declined by 39% during the first six months of the year compared with the same period last year, to 11 from 18. Serious assaults on staff decreased by 5.2%, while serious injuries to inmates decreased by 2%. At the same time, correction officers have used force against inmates 660 times during the past six months, compared with 614 during the period last year.

Mr. Horn ascribed the increasing use of force to a more potent pepper spray that the department has urged guards to use more often against inmates. He said that previously, guards were more reluctant to use pepper spray against inmates because it was less concentrated and not as effective.

Now, guards are also allowed to spray for longer — up to three seconds, compared with a one-second limit under previous guidelines. “We believe it is a very effective tool,” Mr. Horn said, emphasizing that the use of force is used to keep inmates safe.

Yesterday, speaking at the graduation of a new class of correction recruits, Mr. Horn also said new initiatives such as surveillance cameras in jail hallways have helped to keep violence down.

Like the police department, the Department of Correction has faced problems recruiting new officers. Yesterday, 90 recruits graduated, compared with about 100 in two previous classes this year. Mr. Horn said yesterday that the department had missed its recruiting targets by about 350 in the last fiscal year.