diff --git a/cara/monte_carlo/data.py b/cara/monte_carlo/data.py
index 250bd6e6..4415efbb 100644
--- a/cara/monte_carlo/data.py
+++ b/cara/monte_carlo/data.py
@@ -60,7 +60,10 @@ virus_distributions = {
 }
 
 
-# From CERN-OPEN-2021-04 and refererences therein
+# From:
+# https://doi.org/10.1080/02786826.2021.1890687
+# https://doi.org/10.1016/j.jhin.2013.02.007
+# https://doi.org/10.4209/aaqr.2020.08.0531
 mask_distributions = {
     'Type I': mc.Mask(Uniform(0.25, 0.80)),
     'FFP2': mc.Mask(Uniform(0.83, 0.91)),