diff --git a/README.md b/README.md
index 512c8657ce9366d4ee564b00f2e096c392973791..1906e1c086fbc4a6c54f8327c46b47bc79cde840 100644
--- a/README.md
+++ b/README.md
@@ -30,6 +30,10 @@ so we're targeting filter material able to block particles with minimum ~0.25um
    - Collison nebulizer - generally a pneumatic atomizer [^collisonNebulizer]
    - ![](henderson_apparatus.png)
 - mobile henderson appartus 1968 [^henderson]
+- challenge particles
+    -   Bacillus atrophaeus : 0.95-1.25um, 10^7 cfu in challenge [^DIYmask]
+    -   Bacteriophage MS2 MCIMB10108: single strange RNA ~20nm diameter, 10^9 pfu in challenge [^DIYmask]
+    -   E. Coli phi174 [^nelson]
 
 performance difference between pleated hydrophobic filters and electrostatic filters??
 
@@ -39,4 +43,5 @@ performance difference between pleated hydrophobic filters and electrostatic fil
 [^henderson]: [Mobile Henderson apparatus](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2130736/pdf/jhyg00097-0062.pdf)
 [^collisonNebulizer]: [Collison Nebulizer](https://arxiv.org/pdf/2001.04498.pdf)
 [^stability]: [Aeorsol and surface stability of HCoV-19 compared to SARS-CoV-1](https://www.medrxiv.org/content/10.1101/2020.03.09.20033217v1.full.pdf?__cf_chl_jschl_tk__=e5c36168cc7402ab11178538c0ef5fdd2b8c8cbd-1584464414-0-AfohFaEsIDDjmqQ6YEGAmPrX14_fYwDCairndak5B58MzTDypSP7pDWh1jiStM9Xr_70C8k2y_R4fLiH9CFS5W4Y8Q3u3jMABGFCptEt34BXergTOg1G120143eK6Wib1g34BQflq9yKQ5kuuKjU7or6pxdvmtOIsI45onaedQTa8Ioxpo-tMjo83zh6akUTE8BynDs6RQSLB-Ey8zsmuVgG4KLh38sb-sNoxZGizcaBZ7QmCUQVEZlzBKlbKMpAqBCvSOizA0FD2LgWVBFa1UlJ1DYF-l0_-hC0xFvK3lsbZHriRizQA_KZqylQ9l9QFMQo3642MuAGGRCbzwkcC_0)
-[^aerodynamics]: [Aerodynamic Characteristics of RNA concetration of SARS-CoV-2](https://www.biorxiv.org/content/10.1101/2020.03.08.982637v1.abstract)
\ No newline at end of file
+[^aerodynamics]: [Aerodynamic Characteristics of RNA concetration of SARS-CoV-2](https://www.biorxiv.org/content/10.1101/2020.03.08.982637v1.abstract)
+[^nelson]: [Nelson Laboratories: Viral Filtration Efficiency (VFE) at increased challenge level](https://gitlab.cba.mit.edu/pub/coronavirus/tracking/uploads/164d119b5a7a5751c685a6677378668a/ER_2235600_Attachment_5_Viral_Filtration_Efficiency.pdf)