EXPLAINER: How phones can alert you to COVID-19 exposure

More than 8.1 million people in the U.S. have turned their iPhones and Android devices into pandemic contact-tracing tools, but it hasn't been of much use when their neighbors, classmates and coworkers aren't on the same system.

Apple and Google co-created “exposure notification" technology to alert phone users if they spent time near someone who tests positive for the coronavirus, so they know to get tested, too.

It's built with tight privacy restrictions to maintain users' anonymity, but the tech companies have left it to each state's public health authority to decide whether to use it. So far, 16 U.S. states, plus Guam and Washington, D.C., and more than 30 countries have made the exposure notification system available to their residents.

HOW DOES IT WORK?

The technology relies on Bluetooth short-range radio signals to detect when two phones are in close proximity for long enough for someone to likely transmit the virus. Most states measure that close contact as within 6 feet for at least 15 minutes in a day.

Those wireless encounters — the kind that might happen between strangers on a train or in a crowded store — are randomly generated into keys and temporarily logged in a way that doesn't reveal a person's identity or geographic location.

When one person tests positive for the virus, and state health workers verify the diagnosis, others who recently spent time near the infected person get an automatic alert. That also comes with advice from your state health agency about how to get tested and avoid spreading the disease.

WHERE AND HOW CAN YOU USE IT?

In Colorado, Connecticut, Maryland and Washington, as well as Washington, D.C., iPhone users don't have to download an app, but will have to adjust their phone settings to consent to the tracking. Android...