The state Department of Health on Tuesday launched its version of a contact tracing app for smartphones that can inform participants when they come into “close contact” with other participants who later test positive for the coronavirus, so those who have been exposed can protect themselves and others, in case they too become infected.

COVID Alert PA is free from the Apple App store or the Google Play store, and had been downloaded more than 28,000 times by 7:45 p.m. Tuesday.

The Apple-Google system creates anonymized digital keys that are exchanged through Bluetooth technology when participants are in close proximity without enabling the department to track participants’ identities or locations.

The procedures that follow do not reveal the identities of those who are infected to those who are exposed to infection, according to department officials, vendor representatives and university consultants on a conference call Tuesday.

The app will supplement traditional contact tracing, especially when people who test positive don’t know the names of those they may have exposed — on public transportation, at a sports event or in line at a grocery store, according to officials.

The app also can provide exposure notifications to people of whom those who test positive have forgotten to mention to case investigators. And it can provide exposure notifications to people who might not answer the phone when called by contact tracers.

Traditional contact tracers for the state connect with about 70 percent of those who’ve been potentially exposed, Gov. Tom Wolf said Tuesday.

Users’ apps will register with one another when mutual participants are within 6 feet for 15 or more minutes, officials said.

“Proximity is the only thing measured between phones, not location,” a department news release stated.

If one of the participants later tests positive for COVID-19, the department will contact that person as part of its normal case investigation procedures and ask if that person has downloaded the app, according to department spokesman Nate Wardle.

If the person is willing, the department will then provide a six-digit code, which will “share (the infected person’s) Bluetooth chirps with us,” Wardle wrote in an email.

Those digital “chirps” are uploaded so that the phones of participants who have been in close contact within 14 days of the positive test will be notified and provided with suggestions to quarantine and take other protective steps.

The system tells the department how many app users receive exposure notifications, but does not tell the department the identity of those exposed, according to the news release.

App users must be at least 18 years old.

Pennsylvania has partnered with the state of Delaware, which launched its version of the app last week, so that the apps of both states will be “interoperable,” officials said.

Pennsylvania also expects to partner with New Jersey and New York in similar fashion, so that notifications between participants from the various states will be “seamless,” officials said.

Pennsylvania’s partners in the venture include NearForm, the University of Pennsylvania’s Center for Health Incentives and Behavioral Economics and the Massachusetts Institute of Technology’s Lincoln Laboratory, Computer Science and Artificial Intelligence Laboratory and Internet Privacy Research Institute.

The setup is modeled on one pioneered in Ireland, officials said.

“This is an innovative new tool in the fight against COVID-19,” said Health Secretary Dr. Rachel Levine during the news conference.

“I think it will be a real big step for all of us,” Wolf said.

Mirror Staff Writer William Kibler is at 949-7038.