diff --git a/cara/apps/calculator/templates/calculator.form.html.j2 b/cara/apps/calculator/templates/calculator.form.html.j2
index 66207acf..c1c2e935 100644
--- a/cara/apps/calculator/templates/calculator.form.html.j2
+++ b/cara/apps/calculator/templates/calculator.form.html.j2
@@ -153,11 +153,32 @@
       <label for="mechanical_ventilation">Mechanical</label>
     </div>
     <div>
-    <input type="radio" id="natural_ventilation" name="ventilation_type" value="natural_ventilation" data-enables="#DIVnatural_ventilation">
+    <input type="radio" id="natural_ventilation" name="ventilation_type" value="natural_ventilation" data-enables="#DIVnatural_ventilation" data-toggle="modal" data-target="#warning_modal">
       <label for="natural_ventilation">Natural</label><br>
     </div>
     </div>
 
+    <!-- Modal -->
+    <div class="modal fade" id="warning_modal" tabindex="-1" role="dialog" aria-hidden="true">
+      <div class="modal-dialog" role="document">
+        <div class="modal-content">
+          <div class="modal-header">
+            <h4 class="modal-title">Natural Ventilation</h4>
+            <button type="button" class="close align-self-center" data-dismiss="modal" aria-label="Close">
+              <span aria-hidden="true">&times;</span>
+            </button>
+          </div>
+          <div class="modal-body">
+            Single-sided ventilation is assumed in the model and is typically effective for room depths up to 5.5 m from the opening, or up to a depth 2.5 x the ceiling height. 
+            If these conditions are not met, the air exchange might not be homogenous producing an artificially lower risk further away from the window.
+            <br>
+            <br>
+            <img src="/static/images/nat_vent_dimensions.png" id="nat_vent_image">
+          </div>
+        </div>
+      </div>
+    </div>
+
     <div id="DIVmechanical_ventilation" class="tabbed" style="display:none">
         <div class="split">
           <div>
diff --git a/cara/apps/static/css/style.css b/cara/apps/static/css/style.css
index 5db3e6ac..1009b72c 100644
--- a/cara/apps/static/css/style.css
+++ b/cara/apps/static/css/style.css
@@ -89,6 +89,10 @@ body {
   height: 4em;
 }
 
+#nat_vent_image {
+  height: 10em;
+}
+
 .cara_home_image {
   height: 18rem;
   padding: 1rem;
@@ -210,6 +214,10 @@ footer img {
     font-size: 2rem;
   }
 
+  #nat_vent_image {
+    height: 15em;
+  }
+
   .header_text {
     font-weight: bold;
   }
diff --git a/cara/apps/static/images/nat_vent_dimensions.png b/cara/apps/static/images/nat_vent_dimensions.png
new file mode 100644
index 00000000..b6275c59
Binary files /dev/null and b/cara/apps/static/images/nat_vent_dimensions.png differ