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  Updated: 10 May 2020


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Design tips for garage and parking houses on the use of CO and NO2 detectors

Carbon monoxide (CO petrol)/nitrogen dioxide (NO2 Diesel)
Acceptable concentration of carbon monoxide in garages, tunnels, etc. are governed by the overall BBR 1999 and the Swedish Construction Standard 1980 Edition 2, 3:67.
The limit is set to 50

Strangely enough, there are no rules in BBR 1999 or in the Swedish Construction Standard for acceptable concentrations of nitrogen dioxide but the rules for Exposure Limits (AFS 1996:2) are very strict.

Carbon monoxide CO has a threshhold level (a limit for exposure during the working day for ambient air monitoring) of 35 ppm.
If the source is exhaust fumes, the threshhold level is 20 ppm.

The Short-term value (a recommended value, which consists of time-related average of exposure over a reference period of 15 minutes) is 100 ppm, except for exhaust fumes.

Nitrogen dioxide NO2 has a threshhold level for emissions at 1 ppm, and in cases of no exhaust fumes, 2 ppm.
Top limit where there are no exhaus fumes is set to 5 ppm.

The Swedish building standard states that if the CO concentration exceeds 50 ppm the fans must rotate at full speed and logically, this should also apply to a concentration of nitrogen dioxide of 1 ppm.

Sensor Placement:
Carbon monoxide has a slightly lower density than air but is still close to the air density that can be easily affected by  draft.

It is normally said that gas with a lower density (lighter gas) rises towards the ceiling and that you should adjust the sensor placement in these cases, but when the gas density is very close to that of the air, it must be taken into account that it is very easily influenced and can end up anywhere in the room.

In these cases, the sensor is best placed at breathing height.

On/off 3 threshhold levels for CO


Analogue 4-20mA/0-10Vdc sensors for CO and NO2


 Nitrogen dioxide is heavier than air and sinks in the air, but is also sensitive to air draft.

Recommended height is about 2dm above the floor

Number of detectors that may be needed due to the garage design and number of locations. Some rules of thumb are:

  1. One detector per 400m2

  2. If no data exists on the size of the garage and its surface, each car requires 25m2 incl. drive-in and drive-out surfaces.
    It is advisable that the fans be run at low speed if there are cold cars in the garage in order to ventilate any gasoline vapors. Where nitrogen dioxide occurs, the following applies if the source is emissions from diesel engines or LPG-powered ice-makers on the ice rinks.

NO2 -alarm in the garage:

  • 1 ppm warning: Fan starts or is run from half to full speed

  • 5 ppm alarm: Threshold level

Carbon Dioxide (CO2) and Combustion Sensing

Carbon dioxide is one of the most plentiful byproducts of the combustion process used by automobiles, trucks and fuel-driven appliances.

For properly operated and maintained equipment, CO2 will be produced in quantities 100 times or greater than any other combustion byproduct that is considered harmful including carbon monoxide, hexane, nitric oxide or nitrogen dioxide.

It is also important to note that the catalytic converters used in all vehicles produced in North America will convert almost 100% of carbon monoxide and hexane into CO2.

The figure to the right shows a breakdown of the byproducts of combustion by volume (in ppm) for an automobile at idle.

Note that nitrogen and oxygen are also produced in the combustion fumes but are not considered harmful byproducts.


Because CO2 is the largest byproduct of combustion, it can be used to indicate the presence of combustion byproducts. For example the US Bureau of Mines has recommended that carbon dioxide can be used as an index for measuring and controlling diesel pollutants.

Also, the 1995 ASHRAE Applications handbook states: “Control (of combustion fumes) by instrumentation can be simplified by monitoring CO2 levels, as studies have shown the relationship between various diesel engine pollutants and CO2

The chart below provides the 8 hour Threshold Limit Value established for the most common combustion byproducts.

Assuming the proportion of CO2 production in byproduct production is similar to the above breakdown, the chart below shows the level of CO2 that would have to be reached for the TLV of other contaminants to be achieved.

Gas By-Prod.

TLV concentration

CO2 Level Equiv.

Carbon Dioxide

5,000 ppm

5,000 ppm


500 ppm

91,700 ppm

Carbon Monoxide

50 ppm

4,580 ppm

Nitric Oxide

25 ppm

4,580 ppm

Nitrogen Dioxide

5 ppm

7,860 ppm

In actual applications where CO2 is used to control  combustion byproducts, the CO2 threshold should be considerably lower than indicated above in order to provide a significant safety margin and to consider the wide range of vehicles that might be operating in a facility such as parking garages.

The US Bureau of Mines has suggested that 1,300 ppm of CO2 is a good control level for diesel equipment in mines.

In parking garages, maximum levels of 700-800 ppm ensure that ventilation systems are responsive to combustion fumes and that the possibility of buildup of other contaminants is negligible.


  1. H.D. Daniel Jr, “Carbon Dioxide As An Index Of Diesel Pollutants”, US Bureau Of Mines, U.S. Department Of The Interior, IC-9324, 1992

  2. ASHRAE 1995, ASHRAE Applications Handbook 1995, American Society Of Heating & Refrigeration Engineers, page 12.15, Control by Contaminent Level Monitoring



CDR 142 CO2-givare med specialprogram för lastutrymmen


CDK CO2-givare med specialprogram för lastutrymmen
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