Meaning of a reaction being "too slow to be biologically significant"?

[asimov42]

I'm wondering if there's a clear definition of the phrase "too slow to be biologically significant"? (in this case the reaction of the auto-oxidation of $$NO$$ to form $$NO_2$$) Does this simply mean the reaction occurs with a very low probability? Presumably the reaction does still occur - I'm wondering if there's a clear technical defintion.

Thanks.

 

[.Scott]

Per CDC guidelines, the National Research Council's Short-term Public Emergency Guidance Level for a 24-hour exposure to NO2 is 0.04ppm (for inhalation).
The point is: Only the dose makes the poison (at least for NO2). And a thank you to 16th Century Paracelsus.