diff --git a/caimira/models.py b/caimira/models.py
index 87f98776..140f3935 100644
--- a/caimira/models.py
+++ b/caimira/models.py
@@ -1214,7 +1214,7 @@ class CO2ConcentrationModel(_ConcentrationModelBase):
     def removal_rate(self, time: float) -> _VectorisedFloat:
         # 0.25 is a minimal, always present source of ventilation, due
         # to the air infiltration from the outside.
-        return self.ventilation.air_exchange(self.room, time) + 0.25
+        return self.ventilation.air_exchange(self.room, time) + 1e-6
 
     def min_background_concentration(self) -> _VectorisedFloat:
         """
diff --git a/caimira/tests/models/test_co2_concentration_model.py b/caimira/tests/models/test_co2_concentration_model.py
index 8c20a994..d1a8a6db 100644
--- a/caimira/tests/models/test_co2_concentration_model.py
+++ b/caimira/tests/models/test_co2_concentration_model.py
@@ -8,7 +8,7 @@ from caimira import models
 def simple_co2_conc_model():
     return models.CO2ConcentrationModel(
         room=models.Room(200, models.PiecewiseConstant((0., 24.), (293,))),
-        ventilation=models.AirChange(models.PeriodicInterval(period=120, duration=120), 0.),
+        ventilation=models.AirChange(models.PeriodicInterval(period=120, duration=120), 0.25-(1e-6)),
         CO2_emitters=models.Population(
             number=5,
             presence=models.SpecificInterval((([0., 4.], ))),