Better think before you spit.

The MBTA is planning to use DNA testing to track down people who spit on T employees, said MBTA Transit Police Chief Paul MacMillan.

About 30 percent of assaults on T workers involve someone spitting on the workers, MacMillan said. The number of these kinds of assaults has increased, he said.

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The incidents often occur due to fare disputes, he said.

“We’re saying to the offender, We’re not going to tolerate this,” MacMillan said. “We’re going to use any means necessary to find you, and prosecute you.”

Employees can turn in a spit sample – whether from their clothes or from a tissue they used to wipe themselves. The spit will be run through a database of individuals who have been convicted of crimes.

The DNA testing program was reported in today’s editions of the Boston Herald.

“We’re also saying to our employees, ‘We value you,’” MacMillan said.