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Several symbols of the smell of water.
Six symbols of water odor

Odor grade refers to several grades of odor intensity determined by human sense of smell according to olfactory threshold. Generally, the odor intensity is divided into six grades: 0 tasteless, 1- detection threshold, 2 cognitive threshold, 3 obvious detection, 4 strong odor and 5 sudden odor. If the ammonia concentration is lppm, it is rated as 2.5, 2ppm is 3, and 5ppm is 3.5. The odor level of various odors exceeds 2.5-3.5, indicating that the atmosphere is polluted by odor and needs to take preventive measures.