As we all know, harmful gases play a negative role in our society and our environment. Are these gases playing as abiotic or biotic factors in our lives?
Bez is correct, biotic and abiotic refer to the nature of the agent performing the stress.
Now the example "shade" is trickier; because it depends on the perspective you choose.
There's always a context. Does the element creating the shade matter in your context? If so, and this element happens to be animated/living: stress = biotic (though you should reference at some point that the object at hand is animated).
If it doesn't matter what object generates the stress since you are only interested in the effect of the shade on a system: shade = abiotic
Pipeline: Analyze the context, choose and communicate your choice of perspective/angle and then remember what biotic and abiotic literally mean.
Coming back to "harmful gases": What is your context meaning harmful for whom? Specify that first. Carbon Dioxide is an example of something that may be deleterious at a specific concentration for one system and at the same time beneficial for another.
Does it matter how these gases were produced? CO2 may be a product of an animated system's metabolism and thus may be called abiotic. It may also be a product of oxidation of a multitude of carbohydrates.
I don't know of an example where Carbon Monoxide is produced by living things.
You could of course circumvent this discussion by analyzing what harmful effect the gases have on your system of interest. For example: they generate respiratory stress on humans, and then you use the effect of the gases on the system to characterize them.