Study by Dr. med. Günther Ritzel
Preventive medicine, as part of social medicine, by its very nature concerns larger populations and presents findings collected from them for discussion compared with untreated or sham-treated controls. The present study is an attempt to better identify, through the detection of the impact of a physical measure, i.e. artificial humidification, the development of certain adverse health effects that are grouped under the collective term of the common cold. The marked seasonal dependence of the frequency of so-called common cold symptoms has already been discussed by several authors and from different points of view. Frequent bouts of common cold to large-scale influenza epidemics occur in temperate climates during the winter. The same is true for other microbial diseases, such as those with pneumococcal and meningococcal pathogens (1, 2). Often, the success of targeted prophylactic measures, i.e. active vaccination, is likely to fail because the dominant pathogen of the common cold changes from year to year or from “wave” to “wave”. Moreover, not all types of viruses that cause colds are sufficiently known to enable the production of a vaccine.
Taking into account the above-mentioned, annual accumulation in the winter months and knowing that the mucous membranes of the oral cavity and the upper respiratory tract even of healthy people harbour the pathogens of a wide variety of colds, the question of the pathogenetic mechanism that allows the latent state of inapperceptive parasitism to become a manifest disease through the reduction of the body’s own resistance was of interest.
Rogers  already pointed out 40 years ago the inverse proportionality between humidity in living spaces and the frequency of certain infectious diseases; his study, however, was not met with the respective response.
In the following years, the impact of humidity as a partial factor of the room climate was discussed and investigated from pathophysiological clinical and microbiological points of view. Humidity is physically dependent on the prevailing temperature in that as heat increases, the saturation capacity of the air with water vapour increases [4, 5]. For illustration purposes, three temperature values and the approximate maximum water vapour contents to be assigned to them are given: 1m3 air is saturated with 5g at 0°C, with 13g at 15°C, with 23g H20 at 25°C. According to the definition of relative humidity as a percentage of the temperature-dependent maximum water content or saturation value, it becomes evident that at high temperatures (e.g. room temperature compared with winter outdoor temperature of 0°C) an equal amount of water vapour only produces a much lower relative humidity, in our example about one-quarter. The fact that with the process of heating the room, the air becomes relatively dry, is the resulting inevitable consequence. Common central, ceiling and warm air heating systems result in relative humidity values of only 20 to 30% .
Indoor climate conditions subjectively perceived as comfortable correspond to about 50% relative humidity; beyond subjective perception, however, these are objectifiable criteria whose non-observance can lead to colds .
Otologists and paediatricians state that as a result of relative dryness of the air, the cilia, hair-like structures on the mucous membranes of the upper airways, are damaged; this noxa leads to the functional loss of the “physiological barrier” against infection with pathogens of colds. This is because, as is well known, the ciliary movements serve to transport corpuscular elements, which have been absorbed with inhalation, to the outside.
In this context, it has been found that a dry atmosphere, due to the steep concentration gradient between the organism and the room air, initially causes the water content of the thin mucus coating on the cilia to fall below 97%, resulting in an increase in viscosity that causes cilia movements to slacken [6, 8]. Consequences include accumulation and proliferation of animate causes of disease in the respiratory tract, infiltration of even deeper tissue layers, and possibly haematogenous dissemination.
According to these findings, but also for other reasons, correcting the decrease in relative humidity associated with conventional heating methods would appear to be a desirable preventive medical measure. Various groups of experimental microbiologists have observed longer survival of certain pathogens in dry environments.
In contrast, in experiments with mice, influenza virus strains proved to be inactive much more rapidly at a moderate relative humidity (50%), so that droplet infections were considerably less frequent [1, 9]. The same finding applies to the pathogen causing cerebrospinal meningitis in tropical Africa  in human populations and also to animal studies with streptococcus-laden aerosols .
Thus, it appears that not only the impairment of the host’s natural resistance, but also the increase in survival time and thus the increased contagiousness of pathogenic micro-organisms can be manipulated by decreasing the relative humidity below certain values. Therefore, appropriate measures are key in the prevention of certain infectious diseases, among them the most prevalent of all, the common cold.
The rationale for studies such as the one reported below is also evident from medical-sociological and economic points of view: No other pathological condition seems to cost a nation as much as the common cold. In the U.S., the Common Cold Foundation calculated that strike action results in only about 20% of material losses to the industry compared with work absences due to common cold and similar ailments .
It stands to reason that the circumstances of whether or not an infectious disease becomes manifest cannot be discussed from a singular point of view alone, such as the one discussed here, namely relative humidity. Various other factors, for example the immune status of the still healthy study participants, play an equally important role, as do environmental factors such as clothing, meteorological influences and others.
In young children (four- to six-year-olds), exposure to animate causes of disease and thus the development of sufficient immunity has taken place only to a limited extent. Upper respiratory infections have been shown to be more common in children than in older individuals, in part because the body mass acts as a thermal buffer for the internal organs — a fact that is inevitably less present in children, so that their thermal balance is more likely to be subject to disruption. At present, the problem of artificially enriching the air in heated rooms with water vapour is topical, since effective atomisers are available at affordable prices [13, 14]; this is at least the case as long as the method of choice, the supply of air heated and humidified to the appropriate level, has not yet been implemented.
In Basel, over 90% of children of pre-school age attend a nursery school. They generally spend four to five hours there on weekdays. Our study used five so-called double unit nursery schools, i.e. five sites with two separate classrooms each, each of which was attended by an average of 21 children. An effective water atomiser (capacity = 0.5l H20/hour or more) was installed in one of the two classrooms of each double unit nursery school, which was in operation during the times the children were present, so that a mist of very fine water droplets was sprayed by a rotating disc. The effectiveness of the equipment, together with the room temperature, was recorded in graphs using continuous hygrometric measurements.
We would like to express our sincere thanks to the head of the Basel-Stadt nursery schools, Ms Rosemarie Zeltner, as well as to the experts of the responsible departments (Education and Construction) for their understanding and active support.
The comparison classroom of each nursery school, which was equipped in the same way and attended by children from the same neighbourhood, was not artificially humidified and did not receive any air enriched with water vapour like the aforementioned classroom; it was also checked by means of hygrometric measurements carried out over a period of 14 days.
Parents and children were not informed about the objective of the measure. The nursery school teachers of all ten premises were instructed to provide us with written information about absences due to cold symptoms (cough, runny nose, sore throat, fever of unclear origin) within the framework of a morbidity report that had to be submitted to the school management for the attention of the school doctors — a report, which had been in place for a number of years. The study lasted nine weeks (January to early March 1965). We refrained from testing devices whose “effect” is based on so-called natural water evaporation, since only very modest increases in humidity are achieved in this way, depending on the evaporating blotting paper surface used per room unit, up to about 3 relative percent .
The temperature measurements showed an average daily value of 22.2°C for the nursery school classrooms with air humidification, and 21.9°C in the control classrooms. Since the heating, when not remotely controlled, was operated by the same entity, no significant differences between the values, which were high in themselves, were to be expected.
The relative humidity fluctuated by a few percent during the course of the day. Mean values could be calculated from the recorded data submitted to us. They were 49% relative humidity (RH) for the artificially humidified rooms and 40% RH for the control rooms. The hygrographs were calibrated at the beginning of each week.
The attendance days at maximum attendance (planned target) and the days of absence due to a cold are shown in the first table, broken down by nursery school. The results show, with considerable variation, that in the nursery school classrooms artificially enriched to around 50% RH, 3.0% of the theoretically possible attendance days were “missed” due to cold symptoms. The corresponding value for the relatively drier classrooms, on the other hand, was 5.7%.
In summary, it is evident that in the five nursery school classrooms with cold steam nebulisers, whose maximum occupancy during the study period would have corresponded to 6306 attendance days, a total of 195 “days of absence due to a cold” were registered, while in the corresponding five control nursery school classrooms with a maximum of 5910 attendance days, 338 “days of absence due to a cold” were recorded.
|Nursery school unit
||Maximum occupancy (days)
||Absence due to a cold (days)
|A in %
|B in %
Maximum attendance days and days missed due to cold symptoms in ten nursery school units with and without humidification, respectively (total of 232 children and 12,216 days of observation).
The statistical evaluation of these findings was done using the X2 test (Table 2).
|Days of absence
due to a cold
|Nursery school units with artificial humidification
|Control nursery school units
Contingency table — X2 test
With a critical X2 value of 3.84 for 2x = 0.05, X2 was empirically > 46, so that the differences obtained must be regarded as confirmed.
Under the given circumstances, the findings indicate that artificially increasing the RH from 40 to about 50% has an infection-inhibiting effect in nursery school children. It can be considered likely that members of the teaching staff with severe work-related strain on the vocal organs would also benefit from such preventive medical measures.
The dispersion within the nursery school classrooms of the same series (A or B) demonstrated in Table 1 can be at least partially explained by individual differences, e.g. the variable relative inability of the nasal mucous membranes to sufficiently humidify dry breathable air. In addition, the symptoms of mild to moderate colds, which is what we were essentially dealing with, are generally not an absolutely compelling reason to keep a child at home, so that questions of subjective discretion on the part of the mother and nursery school teacher may possibly have a place in this survey as an additional risk of error.
In our opinion, it is essential that the artificial humidification only took place up to a RH level of 50%. Values above 60% can lead to moisture penetration of the clothing, which then conducts heat well and brings with it the risk of cooling the surface of the body. We do not wish to make any premature generalisations about the results obtained, as this is the first observation that seems to provide definitive proof of the health-promoting effect of artificial humidification on a representative human population.
It may be that the rather mild winter during the study period, which precluded abrupt temperature changes when leaving the nursery school classrooms, prevented any possible negative effects of the measure studied; a circumstance that could lead to deviating results in other years. Finally, the totality of requirements for a healthy indoor climate cannot be expressed as a simple formula, so that even solitary measures — in this case, improving the water vapour content of indoor air for young children — can hardly effectively counteract every circumstance that favours infection.