Far to many people use these cheap IR thermometers and have no clue about emissivity. Chances are you are using it for measuring duct temp (you think you are anyway) and metal duct has a very low emissivity so all you are really measuring is the reflected temperature. Even the plastic duct work has a low emissivity and will not be correct.
I encourage anyone who is using one of these types of meters to wrap a piece of copper pipe with some standard black electrical tape. Run the hot water and measure the part which has tape versus just the copper tube. You may be surprised what you see.
IR measurements are complicated and there are a lot of variables that come into play for true quantitative data; even for crude qualitative data you need a reasonable understanding of IR theory.
Also please don't fall into the trap of setting one emissivity and using it for everything with the thinking that the error will cancel out; that couldn't be farther from the truth.
Lastly, a much more accurate measure of air temp would be to simply hang a T or K type thermocouple into the air stream. Most decent Fluke DMM's allow thermocouple inputs. I don't think anyone here needs RTD accuracy.