Official Directions from CSPC on How to Correctly Use Baby Sling Carriers
I am sure you heard about the latest news going around about baby carrier recalls by some of the most popular baby slings due to some previously reported baby deaths. 14 deaths, 12 of them under 4-months, in total were associated with this type of baby sling carrier. The big risk of suffocation is what caused this issue.
Now that over 1 million of baby slings are recalled, The US Consumer Product Safety Commission published official directions about how to wear the baby sling and carry the baby in a correct way. This is to reduce, if not totally eliminate, the risk of suffocation of the infants.
Below you can see the images of the correct carriage directions for baby slings.

In the correct carrying position, note that that face of the baby looks upward, visible by the parent, and no risk of suffocation since it will breath easily. On the other case, if the baby is directed towards the chest of the parent, then its face will be covered and cannot breathe easily, resulting in a lack of airing.

Carrying the baby at a very low position is also not suggested, rendering it uncomfortable and as a result, creating a risk of suffocation.

Creating a pressure on the baby, especially on its face while in the carrier is extremely dangerous and should be avoided by all means. Once you feel it becomes uncomfortable for the baby or for you, just give a break and get it out of the carrier, so that both of you take some breath and rest for a while.
P.S. source of images: cspc.gov

