Interpreting the SAMHE Monitor Readings

What you can see on the monitor

SAMHE Monitor panorama

Numbers

The SAMHE air quality monitors measure carbon dioxide (CO2), total volatile organic compounds (TVOCs) particulate matter (PM2.5), temperature and relative humidity. The levels of each of these are shown as numbers on the monitor display.

SAMHE Monitors record:

Monitor readings

Lights

As well as being displayed as a number, the LEDs (coloured lights) displayed on your SAMHE monitor also correspond to the level of CO2 detected.

Monitor lights

CO2 (carbon dioxide)

Understanding the monitor lights

The table below provides a quick overview of what the LEDs tell you about CO2 levels and what those levels mean for your wellbeing, and gives some recommended actions.

If you'd like to know what a specific LED combination means please consult our full guide to SAMHE monitor LEDs during normal operation.

Monitor CO₂ ReadingLED colourNumber of LEDsDescriptionWhat this means for youActions
0-799 ppmGreen
1 - 2 lightsIndicative of good ventilationVentilation is acting to help maintain a good classroom environment, including reducing the risk that airborne diseases are spread.If CO₂ levels are not rising then you can consider having your windows open slightly less wide or temporarily closing them. Do so gradually and in stages.
800-1499 ppmOrange
3 - 6 lightsPotential for increasingly stuffy/stale airConsider the potential to improve ventilation in your classroom for better health and learning outcomes - this might include having more windows & doors open, opening them more widely, or getting any mechanical ventilation serviced.Consider opening windows and/or doors - higher-level openings first and then, if necessary, also lower-level openings. Continue to monitor the CO₂ levels.
1500-3000 ppmRed
7 - 8 lightsIndicative of inadequate ventilationThere are quite high levels of shared/rebreathed air in your classroom which, if maintained, might lead to poorer learning and health outcomes. Consider the potential to improve ventilation (for example, by having more air flow by opening any windows and doors, or getting any mechanical ventilation serviced).Keep checking that ventilation provision (e.g. windows and doors) is fully operational/open and continue to monitor the CO₂ levels. If consistent, notify school leadership.
3000+ ppmRed + Purple
9 lightsIndicative of inadequate ventilationThere are quite high levels of shared/rebreathed air in your classroom which, if maintained, might lead to poorer learning and health outcomes. Consider the potential to improve ventilation (for example, by having more air flow by opening any windows and doors, or getting any mechanical ventilation serviced).Keep checking that ventilation provision (e.g. windows and doors) is fully operational/open and continue to monitor the CO₂ levels. If consistent, notify school leadership.

How are the thresholds for each range of CO2 concentration determined?

The thresholds have been set based on advice from the UK Government's 'Scientific Advisory Group for Emergencies' (SAGE)

The two most relevant publications from SAGE are as follows:

TVOCs (Total Volatile Organic Compounds) and PM (Particulate Matter)



Why so many details for CO2 compared to TVOCs and PM?

Young boy by open window

The reason we provide more detail on how to interpret carbon dioxide (CO2) measurements is not because it's our favourite indoor air quality metric, it's because the CO2 data is easier to interpret. In most classrooms, people are the main source of CO2 and the CO2 level can be reduced by increasing ventilation. This means CO2 is a good indicator of the relative ventilation level in your classroom (no wonder we talk about it so much!). Read more about how you can use the CO2 data to inform decisions about ventilation in our ventilation guidance for schools.

Your SAMHE monitor also measure two other aspects of your classroom air quality - VOCs and PM.

Health issues linked to VOCs and PM

Cough

Volatile organic compounds (VOCs, the 'T' on your monitor's screen stands for 'total' in recognition of the fact that the monitors are designed to detect all VOCs) and particulate matter (PM, or in our case specifically PM2.5) are important too. Exposure to PM and certain VOCs can be linked to negative health effects (ranging from sore eyes, an irritating cough, to more serious outcomes). However the linkages are statistical, which means that it is hard to infer the effects on any individuals from any particular event. In part this is because measurements of VOCs and PM by their very nature encompass a wide variety of things, and not all of these are equally harmful, or even inherently harmful.

To accurately assess any risk within your classrooms, you would need to identify every single compound in the air, and its level everywhere in your classroom at all times you are there. The SAMHE monitors (indeed, even the latest and greatest scientific equipment!) cannot do that.

What are VOCs and PM and where do they come from?

whiteboard pens eraser

VOCs are gases that have evaporated from certain liquids or surfaces. There are a variety of VOCs and many are human-made chemicals that are used and produced in the manufacture of paints, pharmaceuticals, and refrigerants. Some VOCs are harmful, such as chemicals given off by some cleaning products, paints and new furniture. Some are relatively harmless, for example trees give off VOCs and so does orange juice.

chalk dust coloured

It is a similar story for particulate matter (PM). PM2.5 means all particles (including liquid and solid particles) which are smaller than about 2.5 microns and floating in the air. 2.5 microns is very small - a single hair from your head is more than 30 times as wide as that - so you can only see them with a microscope! Some PM is naturally produced e.g. grains of pollen from plants, dust (which includes soil and dead skin cells suspended in the air), mould spores and salt from sea spray. Other PM is produced by human activities such as driving (from tyre and brake wear and exhaust emissions), construction, agricultural processes and both industrial and domestic fuel burning. In schools, chalk dust can be a source of PM. Sweeping or vacuuming can re-release PM into the air.

It is for these reasons that we need to be more cautious when giving advice about VOCs and PMs. What we do know is that your body does not need to breathe in VOCs or PMs, so it seems sensible to avoid high levels of them in general. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors so ventilation can help dilute them. PM levels are also higher indoors in many situations (though maybe not next to a very busy road). Remember our obsession with using CO2 readings to help make decisions about ventilation.

What levels of TVOC are 'high'?

disinfectant spray

The UK Health and Safety Executive (HSE) publishes Workplace Exposure Limits - upper limits for an acceptable level of exposure - for a lot of chemicals, but have never published limits for TVOCs. Neither does the UK Health Security Agency (UKHSA) but it does publish guidelines for volatile organic compounds in indoor spaces However, that only refers to very specific types of VOCs that your SAMHE monitor is not designed to measure individually. The Approved Document F (which applies to England with equivalent documents published for other parts of the UK) details building regulations 'for the ventilation requirements to maintain indoor air quality' and provides a limit for TVOCs of 300 µg/m3 (for exposures of 8 hours) - this is based on levels recommended by the World Health Organisation - but this comes with the caveat that TVOC: "should not be used as a direct indicator of health". Guidance and standards in other countries suggest different values. As such, we cannot suggest any guideline values when you are looking at TVOC data from your SAMHE monitor.

So what good are the TVOC measurements? You will see your SAMHE monitor does not report TVOCs as concentration but as levels within a relative index 'Ind30', the '30' refers to the model of TVOC sensor used within the SAMHE monitors. So don't worry too much about the absolute values, we suggest that, for each space you investigate, you can look at periods of time when the TVOC measurements are relatively low or high and consider what might have caused that. Look especially for short 'spikes' in the TVOC data and have a think about what might be happening within the space at, or just before, these times; try using the SAMHE Be a detective activity to help you. These spikes might occur when classroom cleaning happens, when people use products like deodorant, or when certain foods are eaten - anything that creates a strong smell might lead to a spike.

What levels of PM2.5 are 'high'?

vacuum cleaner carpet

The UK Health and Safety Executive (HSE) does not include Particulate Matter when publishing Workplace Exposure Limits, and the UK Health Security Agency (UKHSA) does not publish limit values either. This is partly because most of the research linking PM levels to health outcomes has been carried out based on measurements made outdoors (in 'ambient' air). Outdoor PM levels, and the sources of PM outdoors, can be quite different to those indoors. For outdoor air, the UK's Department for the Environment and Rural Affairs (DEFRA) publishes 'Air Quality Objectives' which for PM2.5 specifies an annual mean value of 20 ug/m3 (10 ug/m3 for Scotland) and the World Health Organisation (WHO) Air quality guideline values specify a value of 15 ug/m3 (for a daily average, and 5 ug/m3 for the annual average). From this you can see that there are already mixed messages in relation to PM2.5 levels for outdoor air, whereas our focus is helping you to monitor the indoor air within your school. No guideline values for particulate matter have been published based on measurements of PM2.5 exposures indoors.

So what good are the PM measurements?

When we plot your data for PM2.5 we have included (horizontal) grey lines at 5, 10, 15, and 20 ug/m3 corresponding to the maximum average values (per day or year) recommended by WHO, and those specified by different UK agencies. These values differ quite a lot but if the average levels inside your school remain high relative to these guideline values, or you'd just like to try to understand the readings from within your school a little better, then try using SAMHE Be a detective activity to help you investigate.

Temperature and Relative Humidity


Why measure temperature and relative humidity?

Boy in classroom

Whether we feel too warm, too cool, or comfortable is described as our 'thermal comfort'. Managing thermal comfort is important in providing a suitable environment for school children to learn effectively. However, it is not as simple as just managing the temperature of the air around us. Different environmental and personal factors influence thermal comfort, such as air temperature, the 'radiant' temperature of surfaces in the room, air velocity, humidity, clothing, physical activity, age, and other factors.

We are all different and therefore sense/feel things differently. This makes it very difficult to measure the personal factors influencing thermal comfort objectively. The SAMHE monitors measure two key environmental factors: ambient air temperature and relative humidity (see our Key definitions page).

As well as contributing to thermal comfort, relative humidity is also linked to health, as it determines the survival of dust mites, mould, and other disease-causing organisms.

Higher temperatures and humidity levels have also been linked to higher emission rates of VOCs from new products such as furniture, flooring, carpet, etc.

What is a good temperature?

hand of person turning radiator dial

The UK Government publishes recommendations (BB101: Guidelines on Ventilation, thermal comfort and indoor air quality in schools) on appropriate room temperature. However, these recommendations are for operative temperature which is not the same as the air temperature measured by the SAMHE monitors. Operative temperature is a weighted average of ambient air temperature and mean radiant temperature. For classrooms, the BB101 guidance suggests 'normally' operative temperatures should be around 20°C and maximum operative temperatures of 25°C during the heating season. In classrooms dedicated to pupils with physical or learning difficulties, the guidelines suggest a higher normal operative temperature of 23°C.

The air temperature, as shown on the SAMHE monitors, is only one factor contributing to the operative temperature. It is also influenced by the setup of the classroom. Electrical equipment, radiators, sun radiation and humans have high radiant temperatures and so act to increase the operative temperature above the air temperature. Window surfaces and poorly insulated walls have low radiant temperatures and act to decrease the operative temperature. The relative effects of these can vary day to day.

We suggest that you use the air temperature measurements from the SAMHE monitors as an indication of one factor affecting thermal comfort, but rely more on whether staff and students feel too cold or too hot when deciding whether to change the thermal comfort in a room. That can be done by changing the heating level or changing the opening of windows (which will affect the room ventilation). See our ventilation guidance for schools for more information.

What is a good humidity level?

mould

The BB101 guidance for schools does not recommend a specific relative humidity level and states that short-term exposure to very low or high humidity is not a problem.

The Chartered Institution of Building Services Engineers (CIBSE) provides guidance on relative humidity levels in its TM40 - Health and wellbeing in building services document recommending 40-60% relative humidity in domestic settings & mechanically ventilated buildings and 40-70% elsewhere. (The majority of UK schools fall into the second category). CIBSE recommendations are based on World Health Organisation (WHO) guidelines. Similar to CIBSE recommendations, the UK Health and Safety Executive (HSE) has published Sick building syndrome: Guidance for specialist inspectors, recommending relative humidity levels in the range of 40 to 70% for the workplace environment.

The reasoning behind these recommendations is that low relative humidity levels can cause skin and eye problems and indirectly increase particulate matter such as airborne dust and fibres; whereas relative humidity levels above 70% can result in surface condensation and mould growth. A detailed description of health effects of high moisture levels in the air is given in the WHO guidelines for indoor air quality: dampness and mould.

Find more info on the Resources Hub