Air quality 29.02.2020 12:29 04.03.2020 17:47

From 2019, sensors C-AQ-0001R to C-AQ-0004R are replaced by a new generation of sensors C-AQ-0006R.

On the CFox and RFox buses are available modules for measuring CO2, smoke, volatile compounds (VOC) and RH (relative humidity). Modules from the C-AQ-0001R to C-AQ-0004R are designed to be mounted on the wall or in a flush box in the interior, and their dimensions and external connections (CIB buses) are identical.


General guidelines for the placement of sensors in the interior:

Suitable conditions

  • In places that are the most significant in terms of indoor air quality.

  • About 1 ÷ 2.5 m high above the floor level.

  • At least approx. 1m from the corner of the room.

  • In places where the temperature varies in the range from approx. 10 to 40 °C.

  • Close (but not too close) to the exhaust of air from the room.


Unsuitable conditions

  • Close to the windows.

  • Close to the front door.

  • In areas with limited air circulation as the vestibule, niches, etc.

  • In areas with rapid fluctuations in temperature.

  • In areas with rapid fluctuations in humidity.

  • In places where humidity in the air condensates.

  • In places where people would breathe directly on the sensor.

  • In places where you can find vapours of various chemicals, such as detergents, etc.

  • In areas with a risk of the sensor being splashed with various liquids.



CO2 - when you should use carbon dioxide sensors

A good indicator of indoor air quality is the concentration of carbon dioxide (CO2), where the main source of air pollution are people. With increasing concentration of CO2, the levels of other pollutants also increases, such as various bacteria, microorganisms, ammonia, volatile organic substances, and the like. It is therefore recommended to monitor the concentration of CO2, and on the basis of the values measured to either control the ventilation system, or at least manually to ventilate the internal spaces.


Of course in premises equipped with a ventilation equipment it is recommended to use the CO2 sensor for controlling the current power of the ventilation system.

In comparison with the ventilation systems controlled only on the time basis, the systems controlled by air quality sensors can meet the seemingly contradictory requirements: to minimize the power consumption and simultaneously increase and maintain good quality if air indoors.


The CO2 sensors that measure mainly carbon dioxide content in the air do not detect common air pollution.In areas where other sources of air pollution can occur as well, it is recommended to use such air pollution sensors that are sensitive to various gases polluting the air. In these cases, it is insufficient to control the ventilation only based on the values of the CO2 concentration.


Physical evaporation from the surface of skin also releases volatile organic compounds (VOCs), which are the bearer of odours as well. Two-thirds of these pollutants are comprised of acetone, butyric acid, ethanol and methanol. The rest are acetaldehyde, allyalcohol, acetic acid, amyl alcohol, dimethyl ketone and phenol. As the volume of CO2 evaporated from human skin corresponds with that of other harmful substances, and as the concentration of CO2 is easy to measure, the evaluation of indoor air is done via measuring the CO2 content.This procedure is only applicable in rooms where smoking is not allowed, and where the main sources of emissions of harmful substances are human metabolism, building construction, materials and the equipment of the rooms.


The C-AQ-0001R sensor is designed to monitor the current concentration of CO2. Ventilation control (preferably with heat recovery) by monitoring CO2 concentration is very important for the rapidly growing market for low-energy and passive houses.The importance of timely resolution of moisture occurrence is also increasing. In this respect, the recuperation of humidity can be an answer - see the Chapter Ventilation.


A typical level of CO2 in the atmosphere in the country is 350 ppm, while in towns it is 400 ppm, and in city centres it reaches 450 ppm.


Recommended values for indoor environment (living areas):

The recommended target is <800 ppm (high indoor air quality).

A recommended median value is <1000 ppm (medium to medium-low air quality).

A recommended maximum value is < 1400 ppm (low air quality).




SMOKE – when you should use smoke detectors

Carbon monoxide (CO) is a colourless and odourless toxic gas, which is created especially during incomplete combustion. Exposure to higher concentrations may be very dangerous, as carbon monoxide reduces the ability of blood to carry oxygen, which may cause undetected gradual poisoning. Symptoms at low concentrations are headaches, fatigue, nausea, etc.

These symptoms are often observed even at concentrations below 25 ppm.

In most buildings, the concentration of carbon monoxide is below 5 ppm.

Concentration above that level usually indicate the presence of products of incomplete combustion.

The sources of carbon monoxide are mostly smoking and operation of motor traffic.


When tobacco is burning, a whole range of toxic gases is generated, of which the most important from the toxicological point of view is carbon monoxide - CO. It is a colourless and odourless gas with a high ability to bind to haemoglobin (more than 200-times higher ability than that of oxygen) and it is highly toxic. It prevents the transfer of oxygen in the blood from the lungs to the body, which causes asphyxiation.


Another important toxic gas in terms of negative effects on humans is nitrogen dioxide - NO2. It penetrates very easily from the lungs into the bloodstream, causing problems especially to children and sensitive individuals suffering e.g. from asthma. For them, a harmless concentration of nitrogen dioxide is believed to be ten times lower than for healthy individuals. Nitrogen dioxide irritates mucous membranes and causes burning eyes, breathing problems and headaches.

The SMOKE sensor, the C-AQ-0002R module, should be used for controlled ventilation in rooms frequented by smokers (restaurants and other areas with greater movement of people). It is also suitable for controlling ventilation in houses where smokers live.



VOC – when should you use sensors of volatile organic compounds.

There are many synthetic chemicals and even natural materials referred to as volatile organic compounds (VOC). In buildings there are many sources of these chemicals, such as plastics, furniture, construction materials, various chemical cleansers, polishes, cigarette smoke, and also cooking fumes, rotting substances of organic origin, and such like.

The VOC sensor, the C-AQ-0003R module, can be used e.g. for control of ventilation in kitchen operations, etc.



RH - relative humidity

Humidity actually means the quantity of water vapour contained in the air, and this quantity depends on the pressure and temperature.

Humidity of air indoors is usually expressed as the so-called relative humidity indicated as a percentage.


Absolute humidity of air is determined by the weight of water vapour per unit of air volume. The unit of absolute air humidity is one kg/m3.


Relative humidity is the ratio between the quantity of water vapour contained in the air, and the highest possible amount of vapour at a given temperature. It is expressed as a percentage. Relative humidity is calculated as the quotient of the absolute humidity of air to the highest possible absolute humidity of air at a given temperature.


The higher the air temperature, the more water vapour the air can hold, and conversely - when the air is cooling, the relative humidity increases without changing the absolute quantity of water in the air, and vice versa.


Humidity of air is one of the most important quality parameters of the internal environment, which has a significant impact on people's health.


High relative humidity has a number of unpleasant and dangerous effects, such as the occurrence of mould on the walls, especially in the areas of the so-called thermal bridges. These are areas where for some reason there is a lower thermal resistance of the masonry and therefore the temperature is lower as well, resulting in condensation of humidity in the air. Subsequently the plaster and masonry deteriorate, as well as furniture and other wooden structures, which leads to an impairment of the microclimate associated with health risks. This phenomenon occurs for example in older, badly insulated family homes or in buildings, where old windows have been replaced with new ones, which are considerably tighter, but these buildings do not have adequate ventilation.


The opposite situation occurs especially during the heating season, when the indoor humidity is too low.

Less than 40% relative humidity already causes drying of mucous membranes and the respiratory tract illnesses. This is caused by low outdoor air temperature, as the amount of water vapour it contains is too small. Ventilating the room by e.g. opening the window lets some cold outside air into the warm room, where its temperature increases and it results in a further reduction of the relative humidity. Heating up the air drawn in by the ventilation equipment, or by the heat recovery system, has the same effect. Due to the high efficiency of the heat recovery system, users often let it run continuously, even when it is not needed, e.g. when nobody is in the ventilated area. Heating the cold outside air results in a great decrease in relative humidity of the inlet air, and in addition, the inside air with a higher relative humidity is pushed out from the ventilated space. The effect is a decrease of the relative humidity of the internal air even below 30 %.

This can be improved by making use of additional sources of moisture, such as house plants, aquariums, or using humidifiers.


The recommended relative humidity, which entails a feeling of well-being, naturally together with the temperature around 20 °C, is about 50 %. Such environment has a positive effect on our mucous membranes, which are then more resistant to infections.


Relative humidity can be measured e.g. by the C-AQ-0004R module.