What is the optimal relative humidity (RH) for humans? Opinions differ, but mostly the estimates for “optimal” indoor relative humidity are concerned about the house and not the people living in it. Optimal values for human health fall in between 40 and 70% (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474709). The humidity in a rain forest never falls below 80% and even in the savanna the RH is mostly above 50% if that is any reference of a RH that humans have become adapted to. But mostly people deploy air humidifiers when they observe symptoms and the requirements to relieve those symptoms might be different to the requirements for healthy people. Anecdotal evidence has it, that some types of cough only disappear under a hot shower above 37°C, where the RH is close to 100%. Under those conditions, water will probably condensate inside the airways thus thinning the mucus resulting in a better debris removal and reduced postnasal drip*. However, there is evidence that not all respiratory tract infections benefit from higher humidity, e.g. croup (Finnish: kurkunpääntulehdus) seems unaffected by humidity despite common longstanding believe of the opposite (http://www.cjem-online.ca/v6/n5/p357 versus http://www.ncbi.nlm.nih.gov/pubmed/1906598). Interestingly, the use of both humidifiers and dehumidifiers appears positively associated with childhood asthma (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2966669), which could indicate a hygienical problem with such devices in general. Also for asthma too high humidity is contraindicated, although this recommendation seems to be based mostly on the indirect effect via increased mold and mite infestation or bacterial contamination. I could not find much evidence for a direct negative effect of high RH on asthma and some asthma experts have nothing against a reasonable increase of the RH by means of a humidifier provided that the device is maintained clean and that the humidity is kept within safe limits to ensure that it does not contribute to increased allergen exposure (e.g. http://asthma.ca/corp/services/pdf/asthma_humidifiers_vaporizers2_eng.pdf). For more info about the relationship of relative air humidity and asthma see https://books.google.fi/books?id=ehuYVX2-hWYC&num=10.
The problem with relative humidities between 40-70% is, that they can promote mold growth. However, the situation is a bit complicated. Even 70% RH is not much of a concern if there are no temperature differences between indoors and outdoors. Mold needs water to grow and water condensates when the RH reaches 100%. With relative humidities of 40-70%, that happens when cold surfaces meet warm air (windows, outer walls). When this condensation zone cannot buffer (wrong material) nor conduct the humidity away (airtight materials), you’ll have a problem (see e.g. http://www.hs.fi/kotimaa/a1416111843440, article in Finnish). Therefore indoor mold is a more severe problem in colder climates and “optimal” air humidity for the house differs from season to season being the lowest for the cold season. For countries with cold winters even low relative humidities can cause mold to grow. At -30°C the recommendation is that the RH be not above 15%. That is a level so low that I guess that even otherwise healthy individuals will start to experience health problems*. Estimates are that about 20% of all buildings in Finland have a mold problem (http://uutiset.hometalkoot.fi/component/dpcontentplugin/files/download/2..., article in Finnish).
Due to this, buildings in Finland use nowadays mostly forced circulation to ensure that dry air is constantly blown through all rooms. When the temperature drops to -20°C or lower, the relative air humidity can get indoors as low as 15%. This is so low that most people start to complain about respiratory symptoms and dry, cracking skin. Off they go to buy a humidifier. But do those really make a difference?
There are mainly two different types of humidifiers for home use on the market: those that evaporate water by heating and those that create a water mist by ultrasound (http://en.wikipedia.org/wiki/Humidifier). I found the heating type utterly useless: In order to achieve any appreciable increase of humidity (from the typical winterly 30% or less in Finnish apartments to anything above 40%), I had to reduce the air change rate by partially blocking the air vents that blow the air into the room (like this: https://jeltsch.org/sites/jeltsch.org/files/files/airvent500.png). By doing this the room got unbearably hot because the humidifier not only evaporates water but also acts as a quite efficient heating element (I tried the UFOX 3S: http://www.ufox.fi/ilmankostuttimet with a power of 165W, which can evaporate up to 150 ml per hour.
So the only alternative was an ultrasound evaporator. How well they raise the relative humidity depends mostly on the air change rate (ACH, http://en.wikipedia.org/wiki/Air_changes_per_hour, in Finnish “ilmanvaihtokerroin”). That’s a number that tells you how many cubic meters of air are exchanged per hour for each cubic meter of room space (m3/h/m3). There is some provision here in Finland, that this number has to be at least 0.5, but mostly the real numbers in residential homes are nowadays much larger. However, in the context of this minimum, the health effects on the inhabitants are usually discussed (radon, gassing-out from furniture, etc.), but not the effects on the building (see e.g. here: https://www.rakennustieto.fi/Downloads/RK/RK070304.pdf, article in Finnish).
I have made a quick and dirty calculation of how much water evaporation is needed to keep the RH in a small room above 40% and then checked these values against what I observed when we did use the air humidifier in my daughter’s room to reduce her coughing during the night.
The data points:
What does the evaporation rate need to be if 40% RH is the target, the outside temperature -5°C and outside RH 80%, and if the door is closed**? When heating up this air from -5°C to 21°C, the relative humidity sinks from 80% to 15.6%. To increase it to 40% RH for a room of 25.6m3 requires 114 ml water per hour if the ACH is 1. When our humidifier is set to its maximum, the humidity does not reach 40% (it actually hardly changes at all). In order get the humidity to rise, I have to partially block the forced air ventilation. I experimented with different amounts of blocking and found a setting where the RH raises to above 40% when the humidifier is operating at its max. of ~300ml/hour. That empties the tank in less than a day and indicates that our ACH (ilmavaihtokerroin) must be between 2-3 (with the partially blocked air vents!). According to Finnish standards a room should have at least an ACH of 0.5/h. Like probably most newer Finnish apartments, our home exceeds therefore the minimum ACH severalfold. The rationale for such massive circulation is again to avoid the mold problem.
This shows that the Nordica, which belongs to the high capacity evaporators is in the winter (when you need it) just enough for a small room. Provided you block partially the forced ventilation. The specs state for the maximal room size “50 m2” which is more than four times more than the room I tested it in. The 50m2 value probably takes the official minimum of ACH=0.5 as a basis for calculation. Obviously you can create a mold problem with a humidifier: The problems are the cold surfaces like the window, where water will easily condensate even at relatively low RH. The Nordica has the nice feature to switch off at a predetermined RH (which can be set maximally to 70%).
What about humidifiers with less evaporation capacity? I would guess that they are mostly useless unless you blow the humid air straight into your face. Ultrasound humidifiers need regular cleaning. Otherwise, they’ll start blowing germs into the air and make things worse instead of improving them. And cleaning them is not easy as you cannot use detergents (the detergent leftovers can end up in your lungs (http://www.ncbi.nlm.nih.gov/pubmed/24488371). I guess 70% ethanol would be ok for cleaning if you’d let it evaporate thoroughly before using the device again.
*Strangely enough, some commonly accepted correlations between dry air and respiratory system symptoms (e.g. nosebleeds) are controversial in the literature (e.g. http://dx.doi.org/10.1258/0022215054798032 versus http://www.b-ent.be/articles/2014-10-3-199-Kemal.pdf). When reading the literature, it surprised me that scientists establish only simple associations (e.g temperature and nosebleed incidence), while it is clear that the relationship is between a combination of temperature, humidity and residential ventilation preferences (because people in cold climates are in the winter months inside). The article that shows no correlation between seasonality (Bray et al. 2005) is notably from London/UK, where seasonal temperature changes are significantly smaller than in many other countries with more continental climates. Using the meteorological data is not very relevant either if you want to identify possible effects of indoor climate…
**It was absolutely necessay to close the door of the room that I was humidifying. After 10 hours of operation at maximum capacity, the air reached 40% RH from a starting point of 25%. When opening the door, the RH dropped back fairly quickly within hours.