diff --git a/caimira/apps/__init__.py b/caimira/apps/__init__.py
index da903ca1..57441379 100644
--- a/caimira/apps/__init__.py
+++ b/caimira/apps/__init__.py
@@ -1,4 +1,4 @@
 from .expert import ExpertApplication
+from .simulator import CO2Application
 
-
-__all__ = ['ExpertApplication']
+__all__ = ['ExpertApplication', 'CO2Application']
diff --git a/caimira/apps/simulator.py b/caimira/apps/simulator.py
new file mode 100644
index 00000000..f8de9148
--- /dev/null
+++ b/caimira/apps/simulator.py
@@ -0,0 +1,664 @@
+import dataclasses
+import ipywidgets as widgets
+import typing
+import numpy as np
+
+from caimira import data, models, state
+import matplotlib
+import matplotlib.figure
+import matplotlib.lines as mlines
+import matplotlib.patches as patches
+from .expert import collapsible, ipympl_canvas, WidgetGroup, CAIMIRAStateBuilder
+
+# ventilation=models.MultipleVentilation(
+#             ventilations=[
+#                 models.AirChange(active=models.PeriodicInterval(period=120, duration=120), air_exch=100),
+#             ],
+#     ),
+baseline_model = models.CO2ConcentrationModel(
+    room=models.Room(volume=75, inside_temp=models.PiecewiseConstant((0., 24.), (293.15,))),
+    ventilation=models.SlidingWindow(
+        active=models.PeriodicInterval(period=120, duration=15),
+        outside_temp=models.PiecewiseConstant((0., 24.), (283.15,)),
+        window_height=1.6, opening_length=0.6,
+    ),
+    CO2_emitters=models.Population(
+        number=2,
+        presence=models.SpecificInterval(((8., 12.), (13., 17.))),
+        mask=models.Mask.types['No mask'],
+        activity=models.Activity.types['Seated'],
+        host_immunity=0.,
+    ),
+    CO2_atmosphere_concentration=440.44,
+    CO2_fraction_exhaled=0.042,
+)
+
+
+class Controller:
+    """
+    The singleton thing which is the top-level Application.
+
+    It is responsible for owning the Model data and the Views, and
+    orchestrating event messages to each if the Model/View change.
+
+    """
+    pass
+
+
+ScenarioType = typing.Tuple[str, state.DataclassState]
+
+
+class View:
+    """
+    A thing which exposes a ``.widget`` attribute which is a view on some
+    data. This view is essentially a complex combination of widgets, along with
+    some event handling capabilities, which may or may not be sent back up to
+    the underlying controller.
+
+    We strive hard to keep "Model" data out of the View (and try to avoid
+    storing it at all on the View itself), instead relying on being able
+    to notify, and receive notifications, of important events from the Controller.
+
+    """
+    pass
+
+
+class ExposureModelResult(View):
+    def __init__(self):
+        self.figure = matplotlib.figure.Figure(figsize=(9, 6))
+        ipympl_canvas(self.figure)
+        self.html_output = widgets.HTML()
+        self.ax = self.initialize_axes()
+        self.concentration_line = None
+        self.concentration_area = None
+
+    @property
+    def widget(self):
+        return widgets.VBox([
+            self.html_output,
+            self.figure.canvas,
+        ])
+
+    def initialize_axes(self) -> matplotlib.figure.Axes:
+        ax = self.figure.add_subplot(1, 1, 1)
+        ax.spines['right'].set_visible(False)
+        ax.spines['top'].set_visible(False)
+        ax.set_xlabel('Time (hours)')
+        ax.set_ylabel('CO₂ concentration (ppm)')
+        ax.set_title('CO₂ Concentration')
+
+        return ax
+
+    def update(self, model: models.CO2ConcentrationModel):
+        self.update_plot(model)
+
+    def update_plot(self, model: models.CO2ConcentrationModel):
+        resolution = 600
+        ts = np.linspace(sorted(model.CO2_emitters.presence.transition_times())[0],
+                         sorted(model.CO2_emitters.presence.transition_times())[-1], resolution)
+        concentration = [model.concentration(t) for t in ts]
+
+        if self.concentration_line is None:
+            [self.concentration_line] = self.ax.plot(ts, concentration, color='#3530fe')
+
+        else:
+            self.ax.ignore_existing_data_limits = False
+            self.concentration_line.set_data(ts, concentration)
+        
+        if self.concentration_area is None:
+            self.concentration_area = self.ax.fill_between(x = ts, y1=0, y2=concentration, color="#96cbff",
+                where = ((model.CO2_emitters.presence.boundaries()[0][0] < ts) & (ts < model.CO2_emitters.presence.boundaries()[0][1]) | 
+                    (model.CO2_emitters.presence.boundaries()[1][0] < ts) & (ts < model.CO2_emitters.presence.boundaries()[1][1])))
+                   
+        else:
+            self.concentration_area.remove()         
+            self.concentration_area = self.ax.fill_between(x = ts, y1=0, y2=concentration, color="#96cbff",
+                where = ((model.CO2_emitters.presence.boundaries()[0][0] < ts) & (ts < model.CO2_emitters.presence.boundaries()[0][1]) | 
+                    (model.CO2_emitters.presence.boundaries()[1][0] < ts) & (ts < model.CO2_emitters.presence.boundaries()[1][1])))
+
+        concentration_top = max(np.array(concentration))
+        self.ax.set_ylim(bottom=400., top=concentration_top*1.1)
+        self.ax.set_xlim(left = min(model.CO2_emitters.presence.boundaries()[0])*0.95, 
+                        right = max(model.CO2_emitters.presence.boundaries()[1])*1.05)
+   
+        figure_legends = [mlines.Line2D([], [], color='#3530fe', markersize=15, label='CO₂ concentration'),
+                   patches.Patch(edgecolor="#96cbff", facecolor='#96cbff', label='Presence of person(s)')]
+        self.ax.legend(handles=figure_legends)
+        if 1500 < max(concentration):
+            self.ax.set_ylim(top=max(concentration)*1.1)
+        else:
+            self.ax.set_ylim(top=1550)
+        self.ax.hlines([800, 1500], xmin=min(model.CO2_emitters.presence.boundaries()[0])*0.95, xmax=max(model.CO2_emitters.presence.boundaries()[1])*1.05, colors=['limegreen', 'salmon'], linestyles='dashed') 
+        self.figure.canvas.draw()
+
+
+
+class CO2Application(Controller):
+    def __init__(self) -> None:
+        # self._debug_output = widgets.Output()
+
+        #: A list of scenario name and ModelState instances. This is intended to be
+        #: mutated. Any mutation should notify the appropriate Views for handling.
+        self._model_scenarios = []
+        self._active_scenario = 0
+        self.multi_model_view = MultiModelView(self)
+        # self.comparison_view = ExposureComparissonResult()
+        self.current_scenario_figure = ExposureModelResult()
+        self._results_tab = widgets.Tab(children=(
+            self.current_scenario_figure.widget,
+            # self._debug_output,
+        ))
+        # for i, title in enumerate(['Current scenario', 'Scenario comparison', "Debug"]):
+        #     self._results_tab.set_title(i, title)
+        self.widget = widgets.HBox(
+            children=(
+                self.multi_model_view.widget,
+                self._results_tab,
+            ),
+        )
+        self.add_scenario('Scenario 1')
+
+    def build_new_model(self) -> state.DataclassInstanceState[models.CO2ConcentrationModel]:
+        default_model = state.DataclassInstanceState(
+            models.CO2ConcentrationModel,
+            state_builder=CAIMIRAStateBuilder(),
+        )
+        default_model.dcs_update_from(baseline_model)
+        return default_model
+
+    def add_scenario(self, name, copy_from_model: typing.Optional[state.DataclassInstanceState] = None):
+        model = self.build_new_model()
+        if copy_from_model is not None:
+            model.dcs_update_from(copy_from_model.dcs_instance())
+        self._model_scenarios.append((name, model))
+        self._active_scenario = len(self._model_scenarios) - 1
+        model.dcs_observe(self.notify_model_values_changed)
+        self.notify_scenarios_changed()
+        self.notify_model_values_changed()
+
+    def _find_model_id(self, model_id):
+        for index, (name, model) in enumerate(list(self._model_scenarios)):
+            if id(model) == model_id:
+                return index, name, model
+        else:
+            raise ValueError("Model not found")
+
+    def set_active_scenario(self, model_id):
+        index, _, model = self._find_model_id(model_id)
+        self._active_scenario = index
+        self.notify_scenarios_changed()
+        self.notify_model_values_changed()
+
+    def notify_scenarios_changed(self):
+        """
+        Occurs when the set of scenarios has been modified, but not if the values of the scenario has changed.
+
+        """
+        self.multi_model_view.scenarios_updated(self._model_scenarios, self._active_scenario)
+
+    def notify_model_values_changed(self):
+        """
+        Occurs when *any* value in *any* of the scenarios has been modified.
+        """
+        self.current_scenario_figure.update(self._model_scenarios[self._active_scenario][1].dcs_instance())
+
+
+class ModelWidgets(View):
+    def __init__(self, model_state: state.DataclassState):
+        #: The widgets that this view produces (inputs and outputs together)
+        self.widget = widgets.VBox([])
+        self.construct_widgets(model_state)
+
+    def construct_widgets(self, model_state: state.DataclassState):
+        # Build the input widgets.
+        self._build_widget(model_state)
+
+    def _build_widget(self, node):
+        self.widget.children += (self._build_room(node.room),)
+        self.widget.children += (self._build_population(node.CO2_emitters),)
+        self.widget.children += (self._build_ventilation(node.ventilation),)
+        
+    def _build_population(self, node):
+        return collapsible([widgets.VBox([
+            self._build_population_number(node),
+            self._build_activity(node.activity),
+            self._build_population_presence(node.presence)
+        ])], title="Population")
+
+    def _build_room(self,node):
+        room_volume = widgets.IntSlider(value=node.volume, min=5, max=200, step=5)
+        humidity = widgets.FloatSlider(value = node.humidity*100, min=20, max=80, step=5)
+        inside_temp = widgets.IntSlider(value=node.inside_temp.values[0]-273.15, min=15., max=25.)
+
+        def on_volume_change(change):
+            node.volume = change['new'] 
+
+        def on_humidity_change(change):
+            node.humidity = change['new']/100
+
+        def on_insidetemp_change(change):
+            node.inside_temp.values = (change['new']+273.15,)
+
+        room_volume.observe(on_volume_change, names=['value'])
+        humidity.observe(on_humidity_change, names=['value'])
+        inside_temp.observe(on_insidetemp_change, names=['value'])
+
+        widget = collapsible(
+            [ widgets.VBox([
+                widgets.HBox([widgets.Label('Room volume (m³)'), room_volume],
+                layout=widgets.Layout(width='100%', justify_content='space-between')),
+                widgets.HBox([widgets.Label('Inside temperature (℃)'), inside_temp],
+                layout=widgets.Layout(width='100%', justify_content='space-between')),
+                widgets.HBox([widgets.Label('Indoor relative humidity (%)'), humidity],
+                layout=widgets.Layout(width='100%', justify_content='space-between')),])
+            ], title="Specification of workspace"
+        )
+
+        return widget
+
+    def _build_activity(self, node):
+        activity = node.dcs_instance()
+        for name, activity_ in models.Activity.types.items():
+            if activity == activity_:
+                break
+        activity = widgets.Dropdown(options=list(models.Activity.types.keys()), value=name)
+        
+        def on_activity_change(change):
+            act = models.Activity.types[change['new']]
+            node.dcs_update_from(act)
+        activity.observe(on_activity_change, names=['value'])
+
+        return widgets.HBox([widgets.Label("Activity"), activity], layout=widgets.Layout(justify_content='space-between'))
+
+    def _build_population_number(self, node):
+        number = widgets.IntSlider(value=node.number, min=1, max=200, step=1)
+
+        def on_population_number_change(change):
+            node.number = change['new']
+        # TODO: Link the state back to the widget, not just the other way around.
+        number.observe(on_population_number_change, names=['value'])
+
+        return widgets.HBox([widgets.Label('Number of people in the room '), number], layout=widgets.Layout(justify_content='space-between'))
+
+    def _build_population_presence(self, node):
+        presence_start = widgets.FloatRangeSlider(value = node.present_times[0], min = 8., max=13., step=0.1)
+        presence_finish = widgets.FloatRangeSlider(value = node.present_times[1], min = 13., max=18., step=0.1)
+
+        def on_presence_start_change(change):
+            node.present_times = (change['new'], presence_finish.value)
+
+        def on_presence_finish_change(change):
+            node.present_times = (presence_start.value, change['new'])
+        
+        presence_start.observe(on_presence_start_change, names=['value'])
+        presence_finish.observe(on_presence_finish_change, names=['value'])
+
+        return widgets.HBox([widgets.Label('Population presence'),  presence_start, presence_finish], layout = widgets.Layout(justify_content='space-between'))
+
+    def present(self):
+        return self.widget
+
+    def _build_ventilation(
+            self,
+            node: typing.Union[
+                state.DataclassStateNamed[models.Ventilation],
+                state.DataclassStateNamed[models.MultipleVentilation],
+            ],
+    ) -> widgets.Widget:
+        ventilation_widgets = {
+            'Natural': self._build_window(node),
+            'HVACMechanical': self._build_mechanical(node),
+            'HEPAFilter': self._build_HEPA(node),
+        }
+
+        keys=[("Natural", "Natural"), ("Mechanical", "HVACMechanical"), ("No ventilation", "No ventilation"), ("HEPA Filter", "HEPAFilter")]
+
+        for name, widget in ventilation_widgets.items():
+            widget.layout.visible = False
+
+        ventilation_w = widgets.Dropdown(
+            options=keys,
+        )
+
+        def toggle_ventilation(value):
+            for name, widget in ventilation_widgets.items():
+                    widget.layout.visible = False
+                    widget.layout.display = 'none'
+
+            if value == 'No ventilation':
+                node.dcs_select(value)
+                node.air_exch = 0.25
+                
+                return
+
+            node.dcs_select(value)
+
+            widget = ventilation_widgets[value]
+            widget.layout.visible = True
+            widget.layout.display = 'flex'
+
+        ventilation_w.observe(lambda event: toggle_ventilation(event['new']), 'value')
+        toggle_ventilation(ventilation_w.value)
+        
+        w = collapsible(
+            ([widgets.HBox([widgets.Label('Ventilation type'), ventilation_w], layout=widgets.Layout(justify_content='space-between'))])
+            + list(ventilation_widgets.values()),
+            title='Ventilation scheme',
+        )
+        return w
+
+    def _build_month(self, node) -> WidgetGroup:
+
+        month_choice = widgets.Select(options=list(data.GenevaTemperatures.keys()), value='Jan')
+
+        def on_month_change(change):
+            node.outside_temp = data.GenevaTemperatures[change['new']]
+        month_choice.observe(on_month_change, names=['value'])
+
+        return WidgetGroup(
+            (
+                (widgets.Label("Month"), month_choice),
+            ),
+        )
+
+    def _build_outsidetemp(self, node) -> WidgetGroup:
+        outside_temp = widgets.IntSlider(value=10, min=-10, max=30)
+
+        def on_outsidetemp_change(change):
+            node.values = (change['new'] + 273.15, )
+
+        outside_temp.observe(on_outsidetemp_change, names=['value'])
+        auto_width = widgets.Layout(width='auto')
+        return WidgetGroup(
+            (
+                (
+                    widgets.Label('Outside temperature (℃)', layout=auto_width,),
+                    outside_temp,
+                ),
+            ),
+        )
+
+    def _build_hinged_window(self, node):
+            hinged_window = widgets.FloatSlider(value=node.window_width, min=0.1, max=2, step=0.1)
+
+            def on_hinged_window_change(change):
+                node.window_width = change['new']
+
+            # TODO: Link the state back to the widget, not just the other way around.
+            hinged_window.observe(on_hinged_window_change, names=['value'])
+            
+            return widgets.HBox([widgets.Label('Window width (meters) '), hinged_window], layout=widgets.Layout(justify_content='space-between', width='100%'))
+
+    def _build_sliding_window(self, node):
+        return widgets.HBox([]) 
+
+    def _build_window(self, node) -> WidgetGroup:
+        window_widgets = {
+            'Natural': self._build_sliding_window(node._states['Natural']),
+            'Hinged window': self._build_hinged_window(node._states['Hinged window']), 
+        }
+
+        for name, widget in window_widgets.items():
+            widget.layout.visible = False
+            widget.layout.display = 'none'
+
+        window_w = widgets.RadioButtons(
+            options= list(zip(['Sliding window', 'Hinged window'], window_widgets.keys())),
+            button_style='info',
+            layout=widgets.Layout(height='auto', width='auto'),
+        )
+
+        def toggle_window(value):
+            for name, widget in window_widgets.items():
+                widget.layout.visible = False
+                widget.layout.display = 'none'
+
+            node.dcs_select(value)
+
+            widget = window_widgets[value]
+            widget.layout.visible = True
+            widget.layout.display = 'flex'
+
+        window_w.observe(lambda event: toggle_window(event['new']), 'value')
+        toggle_window(window_w.value)
+
+        number_of_windows= widgets.IntText(value= 1, min= 0, max= 5, step=1)
+        period = widgets.IntSlider(value=node.active.period, min=0, max=240)
+        interval = widgets.IntSlider(value=node.active.duration, min=0, max=240)
+        opening_length = widgets.FloatSlider(value=node.opening_length, min=0, max=3, step=0.1)
+        window_height = widgets.FloatSlider(value=node.window_height, min=0, max=3, step=0.1)
+
+        def on_value_change(change):
+            node.number_of_windows = change['new']
+        
+        def on_period_change(change):
+            node.active.period = change['new']
+
+        def on_interval_change(change):
+            node.active.duration = change['new']
+
+        def on_opening_length_change(change):
+            node.opening_length = change['new']
+        
+        def on_window_height_change(change):
+            node.window_height = change['new']
+
+        # TODO: Link the state back to the widget, not just the other way around.
+        number_of_windows.observe(on_value_change, names=['value'])
+        period.observe(on_period_change, names=['value'])
+        interval.observe(on_interval_change, names=['value'])
+        opening_length.observe(on_opening_length_change, names=['value'])
+        window_height.observe(on_window_height_change, names=['value'])
+
+        outsidetemp_widgets = {
+            'Fixed': self._build_outsidetemp(node.outside_temp),
+            'Meteorological data': self._build_month(node),
+        }
+
+        outsidetemp_w = widgets.Dropdown(
+            options=outsidetemp_widgets.keys(),
+        )
+
+        def toggle_outsidetemp(value):
+            for name, widget_group in outsidetemp_widgets.items():
+                widget_group.set_visible(False)
+
+            widget_group = outsidetemp_widgets[value]
+            widget_group.set_visible(True)
+
+        outsidetemp_w.observe(lambda event: toggle_outsidetemp(event['new']), 'value')
+        toggle_outsidetemp(outsidetemp_w.value)
+
+        auto_width = widgets.Layout(width='auto', justify_content='space-between')
+        result = WidgetGroup(
+            (
+                (
+                   widgets.Label('Number of windows ', layout=auto_width), 
+                   number_of_windows,
+                ),                
+                (
+                    widgets.Label('Opening distance (meters)', layout=auto_width),
+                    opening_length,
+                ),
+                (
+                    widgets.Label('Window height (meters)', layout=auto_width),
+                    window_height,
+                ),
+                (
+                    widgets.Label('Interval between openings (minutes)', layout=auto_width),
+                    period,
+                ),
+                (
+                    widgets.Label('Duration of opening (minutes)', layout=auto_width),
+                    interval,
+                ),
+                (
+                    widgets.Label('Outside temperature scheme', layout=auto_width),
+                    outsidetemp_w,
+                ),
+            ),
+        )
+        for sub_group in outsidetemp_widgets.values():
+            result.add_pairs(sub_group.pairs())
+        return widgets.VBox([window_w, widgets.HBox(list(window_widgets.values())), result.build()])
+
+    def _build_q_air_mech(self, node):
+        q_air_mech = widgets.FloatSlider(value=node.q_air_mech, min=0, max=5000, step=25)
+
+        def on_q_air_mech_change(change):
+            node.q_air_mech = change['new']
+
+        # TODO: Link the state back to the widget, not just the other way around.
+        q_air_mech.observe(on_q_air_mech_change, names=['value'])
+
+        return widgets.HBox([q_air_mech, widgets.Label('m³/h')])
+
+    def _build_ach(self, node):
+        air_exch = widgets.IntSlider(value=node.air_exch, min=0, max=20, step=1)
+
+        def on_air_exch_change(change):
+            node.air_exch = change['new']
+
+        # TODO: Link the state back to the widget, not just the other way around.
+        air_exch.observe(on_air_exch_change, names=['value'])
+
+        return widgets.HBox([air_exch, widgets.Label('h⁻¹')])
+
+    def _build_mechanical(self, node):
+        mechanical_widgets = {
+            'HVACMechanical': self._build_q_air_mech(node._states['HVACMechanical']),
+            'AirChange': self._build_ach(node._states['AirChange']),
+        }
+
+        for name, widget in mechanical_widgets.items():
+            widget.layout.visible = False
+
+        mechanival_w = widgets.RadioButtons(
+            options=list(zip(['Air supply flow rate (m³/h)', 'Air changes per hour (h⁻¹)'], mechanical_widgets.keys())),
+            button_style='info',
+        )
+
+        def toggle_mechanical(value):
+            for name, widget in mechanical_widgets.items():
+                widget.layout.visible = False
+                widget.layout.display = 'none'
+
+            node.dcs_select(value)
+            widget = mechanical_widgets[value]
+            widget.layout.visible = True
+            widget.layout.display = 'flex'
+
+        mechanival_w.observe(lambda event: toggle_mechanical(event['new']), 'value')
+        toggle_mechanical(mechanival_w.value)
+
+        return widgets.VBox([mechanival_w, widgets.HBox(list(mechanical_widgets.values()))])
+
+    def _build_HEPA(
+        self,
+        node,
+    ) -> widgets.Widget:
+        
+        HEPA_w = widgets.FloatSlider(value=node.q_air_mech, min=10, max=500, step=5)
+
+        def on_value_change(change):
+            node.q_air_mech=change['new']
+
+        HEPA_w.observe(on_value_change,names= ['value'])
+
+        return widgets.HBox([widgets.Label('HEPA Filtration (m³/h) '),HEPA_w], layout=widgets.Layout(justify_content='space-between'))
+
+class MultiModelView(View):
+    def __init__(self, controller: CO2Application):
+        self._controller = controller
+        self.widget = widgets.Tab()
+        self.widget.observe(self._on_tab_change, 'selected_index')
+        self._tab_model_ids: typing.List[int] = []
+        self._tab_widgets: typing.List[widgets.Widget] = []
+        self._tab_model_views: typing.List[ModelWidgets] = []
+
+    def scenarios_updated(
+            self,
+            model_scenarios: typing.Sequence[ScenarioType],
+            active_scenario_index: int
+    ):
+        """
+        Called when a scenario is added/removed/renamed etc.
+
+        Note: Not called when the model state is modified.
+
+        """
+        model_scenario_ids = []
+        for i, (scenario_name, model) in enumerate(model_scenarios):
+            if id(model) not in self._tab_model_ids:
+                self.add_tab(scenario_name, model)
+            model_scenario_ids.append(id(model))
+            tab_index = self._tab_model_ids.index(id(model))
+            self.widget.set_title(tab_index, scenario_name)
+
+        # Any remaining model_scenario_ids are no longer needed, so remove
+        # their tabs.
+        for tab_index, tab_scenario_id in enumerate(self._tab_model_ids[:]):
+            if tab_scenario_id not in model_scenario_ids:
+                self.remove_tab(tab_index)
+
+        assert self._tab_model_ids == model_scenario_ids
+
+        self.widget.selected_index = active_scenario_index
+
+    def add_tab(self, name, model):
+        self._tab_model_views.append(ModelWidgets(model))
+        self._tab_model_ids.append(id(model))
+        tab_idx = len(self._tab_model_ids) - 1
+        tab_widget = widgets.VBox(
+            children=(
+                self._build_settings_menu(name, model),
+                self._tab_model_views[tab_idx].widget,
+            )
+        )
+        self._tab_widgets.append(tab_widget)
+        self.update_tab_widget()
+
+    def remove_tab(self, tab_index):
+        assert 0 <= tab_index < len(self._tab_model_ids)
+        assert len(self._tab_model_ids) > 1
+        self._tab_model_ids.pop(tab_index)
+        self._tab_widgets.pop(tab_index)
+        self._tab_model_views.pop(tab_index)
+        if self._active_tab_index >= tab_index:
+            self._active_tab_index = max(0, self._active_tab_index - 1)
+        self.update_tab_widget()
+
+    def update_tab_widget(self):
+        self.widget.children = tuple(self._tab_widgets)
+
+    def _on_tab_change(self, change):
+        self._controller.set_active_scenario(
+            self._tab_model_ids[change['new']]
+        )
+
+    def _build_settings_menu(self, name, model):
+        delete_button = widgets.Button(description='Delete Scenario', button_style='danger')
+        rename_text_field = widgets.Text(description='Rename Scenario:', value=name,
+                                         style={'description_width': 'auto'})
+        duplicate_button = widgets.Button(description='Duplicate Scenario', button_style='success')
+        model_id = id(model)
+
+        def on_delete_click(b):
+            self._controller.remove_scenario(model_id)
+
+        def on_rename_text_field(change):
+            self._controller.rename_scenario(model_id, new_name=change['new'])
+
+        def on_duplicate_click(b):
+            tab_index = self._tab_model_ids.index(model_id)
+            name = self.widget.get_title(tab_index)
+            self._controller.add_scenario(f'{name} (copy)', model)
+
+        delete_button.on_click(on_delete_click)
+        duplicate_button.on_click(on_duplicate_click)
+        rename_text_field.observe(on_rename_text_field, 'value')
+        # TODO: This should be dynamic - we don't want to be able to delete the
+        # last scenario, so this should be controlled in the remove_tab method.
+        buttons_w_delete = widgets.HBox(children=(duplicate_button, delete_button))
+        buttons = duplicate_button if len(self._tab_model_ids) < 2 else buttons_w_delete
+        return widgets.VBox(children=(buttons, rename_text_field))
\ No newline at end of file
diff --git a/caimira/apps/simulator/caimira.ipynb b/caimira/apps/simulator/caimira.ipynb
new file mode 100644
index 00000000..13501bfd
--- /dev/null
+++ b/caimira/apps/simulator/caimira.ipynb
@@ -0,0 +1,59 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "pycharm": {
+     "name": "#%%\n"
+    },
+    "scrolled": false
+   },
+   "outputs": [
+    {
+     "ename": "AttributeError",
+     "evalue": "module 'caimira.apps' has no attribute 'CO2Application'",
+     "output_type": "error",
+     "traceback": [
+      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
+      "\u001b[0;31mAttributeError\u001b[0m                            Traceback (most recent call last)",
+      "Cell \u001b[0;32mIn[1], line 3\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[39mimport\u001b[39;00m \u001b[39mcaimira\u001b[39;00m\u001b[39m.\u001b[39;00m\u001b[39mapps\u001b[39;00m\n\u001b[0;32m----> 3\u001b[0m app \u001b[39m=\u001b[39m caimira\u001b[39m.\u001b[39;49mapps\u001b[39m.\u001b[39;49mCO2Application()\n\u001b[1;32m      4\u001b[0m app\u001b[39m.\u001b[39mwidget\n",
+      "\u001b[0;31mAttributeError\u001b[0m: module 'caimira.apps' has no attribute 'CO2Application'"
+     ]
+    }
+   ],
+   "source": [
+    "import caimira.apps\n",
+    "\n",
+    "app = caimira.apps.CO2Application()\n",
+    "app.widget"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3.9.6 64-bit ('caimira')",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.6"
+  },
+  "vscode": {
+   "interpreter": {
+    "hash": "c77495895472738765eb97c8f848f37a4e60c741d594ab92dd40b6b8f4cac818"
+   }
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}
diff --git a/caimira/models.py b/caimira/models.py
index a9e5ae01..81d3183a 100644
--- a/caimira/models.py
+++ b/caimira/models.py
@@ -1008,6 +1008,7 @@ class _ConcentrationModelBase:
         if not self.population.person_present(time):
             return self.min_background_concentration()/self.normalization_factor()
         V = self.room.volume
+        # RR = self.removal_rate(time) if self.removal_rate(time) != 0 else 0.25
         RR = self.removal_rate(time)
 
         return (1. / (RR * V) + self.min_background_concentration()/
@@ -1210,7 +1211,7 @@ class CO2ConcentrationModel(_ConcentrationModelBase):
         return self.CO2_emitters
 
     def removal_rate(self, time: float) -> _VectorisedFloat:
-        return self.ventilation.air_exchange(self.room, time)
+        return self.ventilation.air_exchange(self.room, time) + 0.25
 
     def min_background_concentration(self) -> _VectorisedFloat:
         """