diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index e83a98ae..b87e423a 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -219,7 +219,7 @@
   <b>Ventilation data:</b> <br>
   <ul>
     <li>Mechanical ventilation = check the flow rates with the concerned technical department.</li>
-    <li>Natural ventilation = the window opening distance is between the fixed frame and movable part when open (commonly used values are window height = 1.6m and window opening = 0.15m). Specify the windows open interval in duration and frequency.</li>
+    <li>Natural ventilation = the window opening distance is between the fixed frame and movable part when open (commonly used values are window height = 1.6m and window opening = 0.15m). In case of periodic opening, specify the the duration (e.g. for 10 min) and frequency (e.g. every 60 min).</li>
     <li>HEPA filtration = the air flow of the device. The following values are based on the different fan velocities of a specific commercial device proposed by the HSE Unit:</li>
     <ul>
         <li>Level 6 (max) = 430 m<sup>3</sup>/h (noisy)</li>