Boston police have issued a safety alert, warning residents about the need to shovel snow away from a car’s exhaust pipe before starting a car after three incidents that left two people dead and two young children injured as residents dug out from the Blizzard of 2013.

A 14-year-old boy suffered fatal carbon monoxide poisoning on Nazing Street this morning when he went inside a running car to keep warm while his father shoveled, police spokesman Cheryl Fiandaca said.

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In the second incident, on Woolson Street in Mattapan this afternoon, a man was found dead inside a car with a tailpipe that investigators believe was blocked by snow, allowing buildup of the deadly gas, she said.

In the third incident, a brother and sister, ages 4 and 7, were found unconscious shortly before 5 p.m. inside a car at 156 Porter St. in East Boston, where they were trying to keep warm. They were rushed to a hospital and treated for carbon monoxide poisoning. Police said the children are expected to survive.

“People have not seen this much snow in a long time. They’re not focused on making sure they clear the snow from the exhaust pipe before putting anyone inside,” said Fiandaca. “Carbon monoxide fills the car pretty quickly…People are not focused on it when they are shoveling.”

In the Nazing Street incident, which happened at about 11:40 a.m., the boy was taking a break from shoveling out from the blizzard.

After the boy and his father had cleared the passenger side of the sedan, the boy got cold and sought refuge in the running sedan whose tailpipe was covered by a snow bank, Boston fire spokesman Steve MacDonald said. The car filled with carbon monoxide, and the father continued to shovel; the boy became sick and stopped breathing.

Firefighters found the boy in cardiac arrest receiving CPR from two neighbors, MacDonald said. The father, possibly because of distress, then went into respiratory arrest, MacDonald said.

The boy was only in the car for 10 to 15 minutes, but the car was covered with snow and the colorless deadly gas could not escape through the muffler, said Jennifer Mehigan, a spokeswoman for Boston EMS.

The boy was revived by the neighbors, paramedics, and firefighters, but was pronounced dead at Boston Medical Center, MacDonald said. The father was also taken to BMC and is in serious condition with a heart issue, Fiandaca said..

“This is definitely a tragic incident and something that we hate to see, especially when we believe it is avoidable,” said Mehigan.

In the second incident, at about 4:40 p.m., a man was found dead inside a car.

MacDonald said firefighters found the man on Woolson Street sitting in a car with the engine running.

“Neighbors told us the guy was in the car since 11 a.m Saturday morning. They broke the window and there was no response,” MacDonald said. The man was pronounced dead at the scene.