EMST is a tax levied on workers, and it is withheld by employers. An employee only pays the tax once per year, even if he has numerous employers in a calendar year.
EMST on a W-2 form stands for Emergency and Municipal Services Tax. It is a tax levied by municipalities in Pennsylvania and is imposed by the municipality in which you work, not the municipality in which you reside.
According to the Payroll-Taxes website, the Pennsylvania Legislature passed Act 222, amending Act 511 of 1965. Essentially, the EMST replaced the Occupational Privilege Tax (OPT) on January 2005.
The maximum amount of the OPT was $10. The maximum amount of the EMST is $52. While the limit increased, municipalities were not required to increase the tax amount, and some maintain the $10 tax rate, as of 2010.