Thousands of people in the Chicago area simultaneously glanced at their cellphones Thursday afternoon to find an Amber Alert had been issued.

That was the first time an Amber Alert was sent on a special channel that’s not affected by phone call or text-message congestion. When an alert is issued, cell tower signals will blanket specific areas with simultaneous alerts. The channel also sends alerts on weather, imminent threats, catastrophic events or national emergencies.

The technology is used sparingly by authorities so as not to desensitize people to emergency situations.

It is primarily used by the National Weather Service to notify people of severe weather. Another alert classification is “Imminent Threat.” Such alerts were sent out in the aftermath of the Boston Marathon bombings. The last type is the “Presidential Alert.” It is meant for a catastrophic event or national emergency — and it has never been used.

Thursday’s Amber Alert was about a 5-year-old boy who was sleeping in his mother’s car when a man stole it Thursday afternoon in Park Ridge. The boy was found safe hours later.

Alerts will show the type and time of the alert, any action you should take and the agency issuing the alert. The message will be no more than 90 characters.

If you own a capable mobile device, it will automatically receive alerts.

To determine if your mobile device is able to receive alerts, go to www.ctia.org/wea and look for a link for your wireless service provider. That’s where you’ll find a list of mobile devices that will receive the alerts on that network.

It is possible to deactivate a phone’s ability to receive all alerts, accept for Presidential Alerts. The deactivation process varies from phone to phone. Individual service providers can provide more details. Authorities encourage people to leave the system activated.

Email: mdudek@suntimes.com

Twitter: @mitchdudek