acute effects, as does exposure to mold (which can lead to chronic respiratory conditions). We might like to purchase monitors for such things but, at present, there are no fully suitable consumer technologies. In the case of airborne mold, there are monitors that draw inferences on the basis of time, temperature, and humidity, but these do not directly measure mold spores themselves. Optical particulate sensors “see” particulates, but cannot discriminate for mold. Monitors for airborne microorganisms, if they ever become available, might help us ward off unpleasant but most often recoverable maladies.
Monitoring capabilities now in the market
Desktop indoor air quality monitors now entering the consumer market commonly incorporate sensors for constituents such as particulates (generally PM2.5 particulate matter-- under 2.5 microns in size), total volatile organic compounds (tVOCs), and carbon dioxide (CO2). Formaldehyde (CH2O) is sometimes broken out from tVOCs and monitored separately. Occasionally humidity, air pressure, and temperature are thrown in. The reasons given for monitoring each of these constituents vary. Such monitors run from about $100 to $300 depending on measurement features and Wi-Fi capability.
Outdoor particulate levels are generally reported on websites as a composite metric. The Air Quality Index (AQI) scale one generally sees reported is an arbitrary composite derived from the levels of ground level ozone (O3), PM2.5 and PM10, carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). It has six tiers, with 0 - 100 being taken as the range generally thought to be satisfactory air quality, 100 - 200 as potentially negatively affecting sensitive groups, over 200 as unhealthy for everyone, and over 300 as constituting emergency conditions. [Ref. 2] High levels of particulate matter trigger chest discomfort, labored breathing, and asthma. However, this is more of an outdoor problem than an indoor problem. Our exposure to PM2.5 and PM10 particulates is generally driven by transient outdoor events: wildfire, dust blown in on the wind, seasonal pollen, or upwind construction or industrial activity. Your local AQI metrics are easily found online and some Wi-Fi-enabled IAQ devices access them.
The extent to which transient levels of outdoor particulates enter our homes is limited. More often, the indoor particulate issues affecting our health and quality of life are allergens well over PM2.5 dimensions: house dust (and the attendant mites that live off of it [Ref. 3]) and pet dander-- both of which are best addressed by regular change-outs of a high quality filter for your home’s HVAC (Heating, Ventilation, and Air Conditioning) system. High Efficiency Particulate Air (HEPA) filters or electrostatic filters bearing a permanent charge are among the best. (If smokers are still present in your household, spend money on adding activated carbon pre-filters to your HEPA or electrostatic furnace filters.)
Portable HEPA air filter devices have gained popularity during COVID. Those also employing UV inactivation of viruses are of particular interest; however, the flow capacity of such small portable devices is generally incompatible with treating more than a single room. Air ionizers are another class of such portable devices which address particulates. Care must be taken that these do not produce low levels of ozone (O3). Some do.
Indoor exposure to total volatile organic components (tVOCs) is generally a transient phenomenon tracing to a specific event. A tVOC sensor reporting spike levels might trace to the use of paint, glue, cleaning products, or various scented products. However, these are transient events likely to be of no impact to long-term health. An exception to this is the case of homes with new flooring, carpets, furniture, or similar products, which can emit tVOCs for a period of time (formaldehyde (H2CO) being a noteworthy constituent). [Ref. 4] Commodity fuel cell type formaldehyde sensors have rather poor sensitivities and may be of limited value.
Some low cost commodity tVOC sensors used in indoor air quality devices can be a bit of a misnomer. They really just algorithmically infer tVOCs from CO2 levels. In any event, the actions to be taken for high CO2 levels (and inferred tVOC levels) are to increase ventilation by opening windows, turning on ventilation fans, or, in the case of larger buildings with more sophisticated HVAC systems, adjusting the air handler to increase the ratio of intake air to recirculated air. Higher cost true semiconductor-based tVOC sensors are available (at the liability of lifetime and calibration concerns), but the actions they would recommend are the same.
Humidity levels are easily measured and strongly tie to personal comfort. Humidifiers are often coupled with furnaces to add humidity to indoor air in the winter (it helps with sinuses and dry skin). Dehumidifiers are seasonally used in certain regions to better keep us cool by allowing more normal levels of perspiration. Knowing the humidity level in one’s home is interesting, but only truly useful if one has a means of adjusting it upward or downward, or is exploring whether to install such capability.
To sum up, tVOCs (only in the case that there is some persistent long-term indoor source) and particulate matter (again, only in the case that there is some long-term indoor source) may very well affect long-term health. This is definitely not the situation with elevated CO2 and humidity. So, does one really need to spend money on an IAQ device to continuously advise one on each of these conditions? To quote a recent review of IAQ devices appearing in the New York Times, “But most of the time, any action you’d take based on an air quality monitor’s readings are things you should do anyway. Open your windows on nice days…” [Ref. 9] There is an appeal of having more information, but is it valuable?
Let’s not forget about radon
Of all the indoor air pollutants we can readily measure, only in the case of radon are there generally accepted statistics that long-term exposure above certain levels can lead to seriously adverse health outcomes: lung cancer. Radon’s radioactive decay resulting in high-energy ionizing radiation accounts for this difference. Since it is generally only diagnosed at an advanced stage, the statistics of lung cancer outcomes are scary. Thus, all homes should be tested for radon. Long-term testing is necessary-- there is a well-known seasonal variation to radon levels. [Ref. 10] In the U.S., 1 in 15 homes measure over the 4.0 pCi/L U.S. Environmental Protection Agency (EPA) guideline recommending mitigation action, with some larger fraction obviously over the 2.7 pCi/L World Health Organization (WHO) guideline. Geology is the single biggest factor, but so is building footprint, tightness of the building insulation and moisture barrier envelope, number of stories (which contributes to indoor/outdoor pressure differential), and possibly even the origin of any cement used in slabs and foundations. The EPA estimates 21,000 people die of radon induced lung cancer each year. [Ref. 10]
The case for high sensitivity real-time electronic monitors
Most all homes have some measurable level of radon. Given the advent of Wi-Fi connected radon detectors from Ecosense, Inc. (San Jose, CA - https://ecosense.io), highly sensitive, real-time consumer monitors, have become available. The EcoQube, for example, displays graphical radon level trends on paired smartphones. The EcoQube reports via Wi-Fi to either an Android or iOS smartphone app that allows visualization of data over days, weeks, and months. The EcoQube offers an unprecedented radon level data rate (updating measurements every 10 minutes) owing to its 30 CPH/pCi/L (counts per hour per pico-Curie per liter) counting sensitivity-- a high rate that results from its patented pulsed ion chamber measurement technology.
We have seen CO2 being suggested as a proxy for other elements of indoor air quality (such as tVOCs), why not use radon the same way? Since it naturally concentrates in buildings due to temperature and pressure differentials, there is a measureable level in most every indoor environment. The actions one might take in response to increasing radon are the same as one might take on the basis of elevated measurements of CO2, tVOCs, or particulates (depending on outdoor conditions): increase ventilation! If radon is trending upward, even if in a range well below the EPA action level, one can open windows or turn on a ventilation fan-- you will simultaneously lower the levels of all other indoor contaminants as well.
The most impactful indoor air quality monitor?
One can argue a sensitive high-data-rate radon detector that graphically outputs trends to a smartphone is the single best compliment to mandated smoke alarms and carbon monoxide detectors. Firstly, one can determine whether one’s home is above the recommended EPA action level of 4.0 pCi/L and, if so, call in a radon professional for a detailed survey and recommendations. Secondly, however, if one’s home is under the EPA action level, trends can be monitored. For example, suppose current radon levels are under, but are rising toward the WHO 2.7 pCi/L threshold, ventilation decisions can be made from the upward slope of the trend line.
If the slope of the radon trend line shown on one’s smartphone is rising, one can think of it as a proxy for other pollutants or pathogens possibly simultaneously accumulating and ventilate accordingly. The effect of opening a window or turning on a ventilation fan will soon be reflected in the radon reading. Why not drive such decisions on the basis of the only common indoor pollutant that is well documented to, over the long term, have potentially serious affects on one’s health?
So, is a real-time high sensitivity radon detector the single best option for an IAQ device after smoke and carbon monoxide detectors? There is a case to be made.
References
1. U.S. fire deaths: https://www.usfa.fema.gov/data/statistics/fire_death_rates.html
2. Indoor CO2 levels: https://www.nist.gov/publications/quit-blaming-ashrae-standard-621-1000-ppm-co2
3. LBNL study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548274/
4. USDA CO2 safety standards: https://www.fsis.usda.gov/sites/default/files/media_file/2020-08/Carbon-Dioxide.pdf
5. Submarine CO2 levels: https://www.irbnet.de/daten/iconda/CIB7571.pdf
6. tVOCs found in homes and their sources: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4057989/
7. Air Quality Index: https://www.airnow.gov/aqi/aqi-basics/
8. Dust mite allergies: https://www.mayoclinic.org/diseases-conditions/dust-mites/symptoms-causes/syc-20352173
9. NYT review of IAQ devices: https://www.nytimes.com/wirecutter/reviews/best-home-air-quality-monitor/
10. Stanley, F.K.T., Irvine, J.L., Jacques, W.R. et al., “Radon exposure is rising steadily within the modern North American residential environment and is increasingly uniform across seasons,” Sci Rep 9, 18472 (2019).
11. EPA Citizens Guide to Radon: https://www.epa.gov/sites/production/files/2016-12/documents/2016_a_citizens_guide_to_radon.pdf
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