Update README.md

README.md

 # COVID-19 Filter Testing
 
-## mask characterization metrics
-- filtration efficiency 
-- fit factor
+## quick lit review
 
-## conventional test methods
+### existing standards
+- 
+
+### mask characterization metrics
+- filtration efficiency: percent efficiency of upstream colony-forming units (cfu) to downstream cfu [^1]
+- fit factor: ratio of concentration of microscopic particles outside to particles that leaked in [^1]
+- microbial penetration value: number of microbes passing through the filter per 10 million microbes in the challenge [^2]
+
+
+### conventional test methods
 - Henderson apparatus: generates closed circuit microbial aerosols from a Collison nebulizer at a controlled humidity 
+   - Collison nebulizer - generally a pneumatic atomizer [^4]
 
-## Resources
-- University of Cambridge [DIY mask efficacy testing](https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic)
-- https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/
+performance difference between pleated hdrophobic filters and electrostatic filters??
+### Resources
+[^1]: University of Cambridge [DIY mask efficacy testing](https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic)
+[^2]: AR Wilkes [The bacterial and viral filtration performance of breathing system filters](https://onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2044.2000.01327.x)
+[^3]: [Mobile Henderson apparatus](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2130736/pdf/jhyg00097-0062.pdf)
+[^4]: [Collison Nebulizer](https://arxiv.org/pdf/2001.04498.pdf)