After Brett Gardner’s son Miller Gardner was found dead due to carbon monoxide exposure during a vacation in Costa Rica, a CO forensics investigator explained why his family didn’t face the same fate.

A toxicology report after his death found high levels of carboxyhemoglobin, an indicator of carbon monoxide in the blood, according to a statement from Randall Zúñiga, the director of the Costa Rica Judicial Investigation Agency. Miller Gardner’s test showed a saturation of 64 percent, he said, adding that anything over 50 percent was lethal.

Brett Gardner’s son died of carbon monoxide poisoning. Carbon monoxide is known as the “invisible killer” because it is a colorless, odorless, tasteless gas or liquid. Here’s how to protect yourself and your family from carbon monoxide poisoning while traveling.