diff --git a/cara/models.py b/cara/models.py
index 80f993d5..6a7a1531 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -762,6 +762,7 @@ class ConcentrationModel:
 
         return (self.infected.emission_rate(time)) / (IVRR * V)
 
+    @method_cache
     def state_change_times(self) -> typing.List[float]:
         """
         All time dependent entities on this model must provide information about
@@ -777,11 +778,18 @@ class ConcentrationModel:
         """
         Find the most recent/previous state change.
 
+        Find the nearest time less than the given one. If there is a state
+        change exactly at ``time`` the previous state change is returned
+        (except at ``time == 0``).
+
         """
-        for change_time in self.state_change_times()[::-1]:
-            if change_time < time:
-                return change_time
-        return 0.
+        times = self.state_change_times()
+        t_index = np.searchsorted(times, time)
+        # Search sorted gives us the index to insert the given time. Instead we
+        # want to get the index of the most recent time, so reduce the index by
+        # one unless we are already at 0.
+        t_index = max([t_index - 1, 0])
+        return times[t_index]
 
     def _next_state_change(self, time: float) -> float:
         """
diff --git a/cara/tests/models/test_concentration_model.py b/cara/tests/models/test_concentration_model.py
index 9470dd56..5e246533 100644
--- a/cara/tests/models/test_concentration_model.py
+++ b/cara/tests/models/test_concentration_model.py
@@ -70,6 +70,7 @@ def simple_conc_model():
 
 @pytest.mark.parametrize(
     "time, expected_last_state_change", [
+        [-15., 0.],  # Out of range goes to the last state.
         [0., 0],
         [1., 0],
         [1.05, 1.],
@@ -80,6 +81,7 @@ def simple_conc_model():
         [2., 1.999],
         [2.1, 2],
         [3., 2],
+        [15., 3.],  # Out of range goes to the last state.
     ]
 )
 def test_last_state_change_time(
@@ -90,10 +92,6 @@ def test_last_state_change_time(
     assert simple_conc_model.last_state_change(float(time)) == expected_last_state_change
 
 
-def test_last_state_change_time_out_of_range(simple_conc_model: models.ConcentrationModel):
-    assert simple_conc_model.last_state_change(3.1) == 3.0
-
-
 @pytest.mark.parametrize(
     "time, expected_next_state_change", [
         [0, 0],