This is what happened.
In the process of Xia Zhou studying Poison Alchemy.
Although the Academy's textbooks or other classical literature about toxins do not provide a detailed explanation of measurement units, it is not difficult to draw a conclusion after reviewing the content learned:
Up to now, the measurement units that Xia Zhou has used when researching or refining toxins have remained within the range of centimeters to millimeters.
Take [Poison Sensing] that only an Expert Level Poison Alchemist can learn for example.
When Xia Zhou does not activate the poison sensing, he observes the toxins with his naked eyes.
At this point, the higher his [Spirit] attribute value, the clearer the poisonous light he will see.
With at least 20 points of spirit attribute value, he can easily distinguish toxin groups with a diameter of more than 3 millimeters.
After activating [Poison Sensing], Xia Zhou can observe more subtle toxin groups at close range.