diff --git a/cara/models.py b/cara/models.py
index 291dcfc1..96120de9 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -419,8 +419,8 @@ class Virus:
     #: RNA copies  / mL
     viral_load_in_sputum: _VectorisedFloat
 
-    #: Ratio between infectious aerosols and dose to cause infection.
-    coefficient_of_infectivity: _VectorisedFloat
+    #: RNA-copies per quantum
+    quantum_infectious_dose: _VectorisedFloat
 
     #: Pre-populated examples of Viruses.
     types: typing.ClassVar[typing.Dict[str, "Virus"]]
@@ -436,20 +436,20 @@ Virus.types = {
         halflife=1.1,
         viral_load_in_sputum=1e9,
         # No data on coefficient for SARS-CoV-2 yet.
-        # It is somewhere between 0.001 and 0.01 to have a 50% chance
-        # to cause infection. i.e. 1000 or 100 SARS-CoV viruses to cause infection.
-        coefficient_of_infectivity=0.02,
+        # It is somewhere between 1000 or 100 SARS-CoV viruses to have 
+        # a 50% chance to cause infection.
+        quantum_infectious_dose=50.,
     ),
     'SARS_CoV_2_B117': Virus(
         # also called VOC-202012/01
         halflife=1.1,
         viral_load_in_sputum=1e9,
-        coefficient_of_infectivity=1/30.,
+        quantum_infectious_dose=30.,
     ),
     'SARS_CoV_2_P1': Virus(
         halflife=1.1,
         viral_load_in_sputum=1e9,
-        coefficient_of_infectivity=0.045,
+        quantum_infectious_dose=1/0.045,
     ),
 }
 
@@ -584,10 +584,10 @@ class InfectedPopulation(Population):
         aerosols = self.expiration.aerosols(self.mask)
 
         ER = (self.virus.viral_load_in_sputum *
-              self.virus.coefficient_of_infectivity *
               self.activity.exhalation_rate *
               10 ** 6 *
-              aerosols)
+              aerosols /
+              self.virus.quantum_infectious_dose)
 
         # For superspreading event, where ejection_factor is infinite we fix the ER
         # based on Miller et al. (2020).
diff --git a/cara/tests/models/test_concentration_model.py b/cara/tests/models/test_concentration_model.py
index 52fb0546..03a042d6 100644
--- a/cara/tests/models/test_concentration_model.py
+++ b/cara/tests/models/test_concentration_model.py
@@ -12,7 +12,7 @@ from cara import models
         {'air_change': np.array([100, 120])},
         {'virus_halflife': np.array([1.1, 1.5])},
         {'viral_load_in_sputum': np.array([5e8, 1e9])},
-        {'coefficient_of_infectivity': np.array([0.02, 0.05])},
+        {'quantum_infectious_dose': np.array([50, 20])},
         {'η_exhale': np.array([0.92, 0.95])},
         {'η_leaks': np.array([0.15, 0.20])},
     ]
@@ -23,7 +23,7 @@ def test_concentration_model_vectorisation(override_params):
         'air_change': 100,
         'virus_halflife': 1.1,
         'viral_load_in_sputum': 1e9,
-        'coefficient_of_infectivity': 0.02,
+        'quantum_infectious_dose': 50,
         'η_exhale': 0.95,
         'η_leaks': 0.15,
     }
@@ -48,7 +48,7 @@ def test_concentration_model_vectorisation(override_params):
             virus=models.Virus(
                 halflife=defaults['virus_halflife'],
                 viral_load_in_sputum=defaults['viral_load_in_sputum'],
-                coefficient_of_infectivity=defaults['coefficient_of_infectivity'],
+                quantum_infectious_dose=defaults['quantum_infectious_dose'],
             ),
             expiration=models.Expiration(
                 ejection_factor=(0.084, 0.009, 0.003, 0.002),
diff --git a/cara/tests/test_infected_population.py b/cara/tests/test_infected_population.py
index 39fbc039..c237e251 100644
--- a/cara/tests/test_infected_population.py
+++ b/cara/tests/test_infected_population.py
@@ -7,7 +7,7 @@ import cara.models
 @pytest.mark.parametrize(
     "override_params", [
         {'viral_load_in_sputum': np.array([5e8, 1e9])},
-        {'coefficient_of_infectivity': np.array([0.02, 0.05])},
+        {'quantum_infectious_dose': np.array([50, 20])},
         {'η_exhale': np.array([0.92, 0.95])},
         {'η_leaks': np.array([0.15, 0.20])},
         {'exhalation_rate': np.array([0.75, 0.81])},
@@ -17,7 +17,7 @@ def test_infected_population_vectorisation(override_params):
     defaults = {
         'virus_halflife': 1.1,
         'viral_load_in_sputum': 1e9,
-        'coefficient_of_infectivity': 0.02,
+        'quantum_infectious_dose': 50,
         'η_exhale': 0.95,
         'η_leaks': 0.15,
         'exhalation_rate': 0.75,
@@ -40,7 +40,7 @@ def test_infected_population_vectorisation(override_params):
             virus=cara.models.Virus(
                 halflife=defaults['virus_halflife'],
                 viral_load_in_sputum=defaults['viral_load_in_sputum'],
-                coefficient_of_infectivity=defaults['coefficient_of_infectivity'],
+                quantum_infectious_dose=defaults['quantum_infectious_dose'],
             ),
             expiration=cara.models.Expiration(
                 ejection_factor=(0.084, 0.009, 0.003, 0.002),