What we want:

to create/come up with an index that we can use to inform the public and roughly give more meaning to the actual measurements we are doing. Would be something similar to air quality index (AQI).
It doesn't have to be perfect - as long as we can have an index or something, it can be helpful to raise awareness about the need of having such value and can later be refined with new information.
Ideally, we can use it to have an idea on ecological impact, adverse health effects, or maybe even degradation time? (from meeting with Cael).

Some info about AQI and how it is calculate

AQI (air quality index) runs from 0 - 500 and measures: Ozone, sulfur dioxide, carbon monoxide, nitrogen dioxide, and PM10 and PM2.5.

Wikipedia has some good info as well about this:

Air quality index

How Is Air Quality Measured?

The main issue with creating an index for microplastics, is that we don't really know what are their adverse effects, or what is a relatively ideal concentration in water, or how much is too much and can have consequences for people.

There are some studies mainly in the ocean that indicate that some fish species and other organisms can ingest MP, but we don't know if there are any adverse effects.
we still don't understand the interaction between MP and other contaminants that could adhere to the surface (or microorganisms).
Interesting article that shows some current limitations/gaps, etc...

https://enveurope.springeropen.com/articles/10.1186/s12302-014-0012-7

Here is some information from the WHO that may be helpful:

https://www.who.int/water_sanitation_health/water-quality/guidelines/microplastics-in-dw-information-sheet/en/

"Studies on absorption indicate that microplastics >150 µm are likely to be excreted directly through faeces. Uptake of smaller particles is expected to be limited, although absorption and distribution of very small microplastic particles including nanoplastics may be higher. Toxicology studies in rats and mice reported some impacts including inflammation of the liver. However, these few studies are of questionable reliability and relevance, with findings reported at very high exposures that would not occur in drinking-water".