40 ILCS 5/12-151.1

Contributions by employees towards annual increase in retirement annuity. Beginning July 1, 1965, there shall be deducted 1/2 of 1% of salary in the case of each employee as his contribution for the annual increase in the basic retirement annuity; provided that beginning January 1, 1976, the rate of deduction shall be 1% of salary. Such deduction shall continue during the entire time the employee is in service and in receipt of salary.