diff --git a/src/pg_rad/physics/fluence.py b/src/pg_rad/physics/fluence.py
index 8d327ed..8bae5f3 100644
--- a/src/pg_rad/physics/fluence.py
+++ b/src/pg_rad/physics/fluence.py
@@ -25,7 +25,7 @@ def phi_single_source(
 
     # Linear photon attenuation coefficient in m^-1.
     mu_mass_air *= 0.1
-    mu_air = 0.1 * mu_mass_air * air_density
+    mu_air = mu_mass_air * air_density
 
     phi = (
         activity