I think I know what you are referring to! Sorry for the delay in responding to your question. Apparently, by mu you mean millimicron. Perhaps because of a limitiation in the formula editor for posting your message, you were not able to write the Greek symbol for micron.
The "m" in front of the above symbol represents one-millionth of a micron, that is, 10^-6 micron.
The symbol IU is much easier to write in this forum, and means "International Unit." Unfortunately, the IU is not really a standardized international unit that can apply to any substance equally. It is not a part of the SI system (metric system of units), that is based on units of ten from the exceeding small to the exceedingly large, and which is firmly standardized across all chemical substances.
For example, 1 IU means something different depending on the substance. Here is a revealing list of well known substances and their different definitions for 1 IU:
- 1 IU for Insulin = 45.5 micrograms pure crystalline insulin.
- 1 IU for Vitamin A = 0.3 microgram of retinol or 0.6 microgram beta-carotene.
- 1 IU for Vitamin C = 50 micrograms of L-ascorbic acid.
- 1 IU for Vitamin D = 0.025 microgram cholecalciferol.
- 1 IU for Vitamin E = 0.667 microgram d-alpha-tocopherol.
The reason for this range of values for the IU is its being based on a measured biological effect for each of these substances. That is, 1 IU has a different magnitude of physiological effect depending on the substance (often a drug, hormone, vitamin, or medicine).
Now, what I am going to say might be a disappointment to you: You cannot relate the millimicron to the IU no matter what substance you are describing. The reason is they are in different categories of measure. The millimicron is a metric system of LENGTH. The IU is based on measures of MASS. To convert one to the other they need to be in the same category of measure, length to length, or mass to mass. One COULD convert quantity of length to a quantity of mass, but only if you first convert either length to mass, or mass to length; but that would require the very inconvenient use of additional data, such as surface area and density of the substance of interest - a consequence too impractical for common use.