"Most workers in Massachusetts have the right to earn and use up to 40 hours of job-protected sick time per year to take care of themselves and certain family members."

(bold emphasis mine)

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"Employers may cap the amount of earned sick time hours accrued at 40 hours, regardless of the additional hours worked by an employee. Once an employee possesses a bank of 40 hours of unused earned sick time, the employer may opt to delay further accrual until the employee draws down the bank of earned sick time
to below 40 hours."

Source