WAYNESBURG, Pa. (AP) — The Pennsylvania prison system is putting in place new policies on mail handling, visits and detection of drones after a month in which about 50 staff reported symptoms that may have been caused by exposure to toxic chemicals.

Gov. Tom Wolf and Corrections Secretary John Wetzel announced the plan Wednesday, after Wolf met privately with staff and union representatives at the State Correctional Institution at Greene in southwestern Pennsylvania. The changes will be imposed over the coming three months.

The state's 25 prisons have been on lockdown and visits have been banned for the past week, as officials have been investigating the spate of illnesses . The cause is suspected to be a clear, odorless substance known as synthetic marijuana that can be concealed in the paper of books and letters.

Inmate mail will be processed outside of the prisons, except legal mail. Legal mail will be copied by staff in the presence of inmates, and the process will be videotaped.

All mail already in prisons will be returned to the senders.

The state will be adopting e-books, and the library system will engage in central purchasing for books and magazines.

The state is expanding its detection of drones and the use of body scanners.

Visiting room staff will be doubled and photos and vending machines won't be allowed for 90 days.

The state is also setting up a hotline for tips about drug smuggling or possession by inmates, staff or visitors.

As of Saturday, the prison system allowed some showers and exercise to resume and general population inmates have begun to be allowed to spend what's described as "modified" time outside their cells.