diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index 24fcc42d..f93fdc00 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -178,6 +178,7 @@
   Enter the data about the area you wish to study. You can find these figures in GIS Portal, or by measuring them yourself.<br>
   <b>Ventilation data</b> <br>
   Enter the data on the available means for venting of indoor spaces. For mechanical ventilation, you should check with a specialist for the air flow or air change rate.<br>
+  For HEPA filtration, you should enter the air flow of the device. As a first estimate, you may use the following values based on the differnet fan velocities of a specific commercial device proposed by the HSE Unit - Level 6 (max) = 430 m^3/h (noisy); Level 5 = 250 m^3/h (ok w.r.t. noise); Level 4 = 130 m^3/h (silent); Level 3 = 95 m^3/h (silent).<br>
   <b>Event data</b><br>
   Enter the total number of occupants in the room and how many of them you assume are infected. We have provided common activity types:<br>
   Office/Meeting = typical scenario all persons seated, talking.<br>