The sulphurous gases that cling at ground level (see The Ground and the Trees) cause a choking effect and are known as The Miasma.
When first taking a breath in The Miasma (don't forget that a creature can hold their breath for a number of minutes equal to 1 + its Constitution modifier with a minimum of 30 seconds, but note that I'm house ruling that if you have taken falling damage on landing the wind has been knocked out of you, and you can't hold your breath), and then every minute afterwards, a creature must make a CON save of DC 14 (in typical atmospheric conditions.)
A success on this roll means that the creature is still breathing OK - for now!
A failure on this roll means that the creature immediately takes 1d12 poison damage, and will continue to do so every minute until they are out of The Miasma, or they fall to 0HP. Death saves taken in The Miasma are made with disadvantage, and if a creature does stabilise then they must be out of The Miasma before they awaken or they will die at this point.
Once a creature is at less than 1/4 of it's full hit points and takes poison damage it becomes poisoned until it is able to breathe clean air again.
Any creature falling to 0HP due to the miasma's poison damage will remain poisoned until they complete a long rest even if they are rescued.
The Miasma is typically 60' thick, although this can vary depending on local conditions.
The ground is a dangerous place to be due to The Miasma alone - add in the likelihood of one of The Gargantuan smelling potential prey, and creatures of the treetops need to get climbing - FAST!