diff --git a/caimira/models.py b/caimira/models.py
index e8de7667..db62293f 100644
--- a/caimira/models.py
+++ b/caimira/models.py
@@ -1132,23 +1132,21 @@ class _ConcentrationModelBase:
             return self.min_background_concentration()/self.normalization_factor()
         
         next_state_change_time = self._next_state_change(time)
-
         RR = self.removal_rate(next_state_change_time)
-        # If RR is 0, conc_limit does not play a role but its computation 
-        # would raise an error -> we set it to zero.
-        try:
-            conc_limit = self._normed_concentration_limit(next_state_change_time)
-        except ZeroDivisionError:
-            conc_limit = 0.
 
         t_last_state_change = self.last_state_change(time)
         conc_at_last_state_change = self._normed_concentration_cached(t_last_state_change)
-
         delta_time = time - t_last_state_change
+
         fac = np.exp(-RR * delta_time)
 
-        return conc_limit * (1 - fac) + conc_at_last_state_change * fac
+        if isinstance(RR, float) and RR == 0:
+            curr_conc_state = delta_time * self.population.people_present(time) / self.room.volume
+        else:
+            curr_conc_state = self._normed_concentration_limit(next_state_change_time) * (1 - fac)
 
+        return curr_conc_state + conc_at_last_state_change * fac
+    
     def concentration(self, time: float) -> _VectorisedFloat:
         """
         Total concentration as a function of time. The normalization
@@ -1260,9 +1258,7 @@ class CO2ConcentrationModel(_ConcentrationModelBase):
         return self.CO2_emitters
 
     def removal_rate(self, time: float) -> _VectorisedFloat:
-        # Setting minimum air exchange rate to 1e-6 to avoid divisions by
-        # zero when computing the CO2 concentration.
-        return np.maximum(1e-6,self.ventilation.air_exchange(self.room, time))
+        return self.ventilation.air_exchange(self.room, time)
 
     def min_background_concentration(self) -> _VectorisedFloat:
         """
diff --git a/caimira/tests/models/test_concentration_model.py b/caimira/tests/models/test_concentration_model.py
index f8f42106..27703a2f 100644
--- a/caimira/tests/models/test_concentration_model.py
+++ b/caimira/tests/models/test_concentration_model.py
@@ -252,8 +252,11 @@ def test_normed_integrated_concentration_vectorisation(
     "known_min_background_concentration", 
     "expected_concentration"], 
     [
-        [0., 240., 240.],
-        [0., np.array([240., 240.]), np.array([240., 240.])]
+        [0., 240., 240. + 0.5/75],
+        [0.0001, 240.0, 240. + 0.5/75],
+        [1e-6, 240.0, 240 + 0.5/75],
+        [0., np.array([240., 240.]), np.array([240. + 0.5/75, 240. + 0.5/75])],
+        [np.array([0.0001, 1e-6]), np.array([240., 240.]), np.array([240. + 0.5/75, 240. + 0.5/75])],
     ]
 )
 def test_zero_ventilation_rate(
@@ -272,4 +275,5 @@ def test_zero_ventilation_rate(
         known_min_background_concentration = known_min_background_concentration)
     
     normed_concentration = known_conc_model.concentration(1)
-    npt.assert_almost_equal(normed_concentration, expected_concentration)
+    assert normed_concentration == pytest.approx(expected_concentration, abs=1e-6)
+    
\ No newline at end of file