M. Schata, J.H. Elixman, W.Jorde
"Association for Applied and Experimental Research of Allergies "
House dust mites and moulds are the most relevant indoor-allergens. Temperature and humidity play an important role as limiting factors for the development of house dust mites and moulds living in the house-dust ecosystem. Under floor heatings reduce the relative humidity more than radiator heatings. In a survey, performed in 21 households, the controlling of house-dust mites and moulds by under floor heating is demonstrated.
Dust mites and moulds belong to the most important indoor allergens, The mites inhabit the dust of mattresses and upholstery, where they produce and deposit the allergen containing mite feces (1). Moulds are ubiquitous, contaminating outdoor as well as indoor air. However, the distribution varies significantly between the different species. Cladospotium herbarum and Alternaria tenuis are the most common outdoor air moulds, while 90% of the first mentioned is found in aeroplancton. It requires a relative humidity of 95% to grow and subsequently can only be found in environments that provide this condition, e.g. houses with Kaltebrucken or humid cellars. Black wall patches make the diagnosis easy.
Aspergillus sp.(see graph) and penicillium sp. are the most common moulds in indoor air, where they find a relative humidity of 70% and lower, which allows them to grow and sporulate. They can especially be found in the house dust . Some moulds, particularly the Aspergillus glaucus group require an even lower relative humidity (below) 60%, These moulds live in symbiosos with the house dust mite and can be found in almost every household (1).
The quantitative occurrence of mites and moulds follows a seasonal patter during the year. In spring, with increasing temperatures and especially increasing relative air humidity, the mites generate, while it takes them about 4 weeks for one generation.
In a hot summer, their number can reach the 50 to 100 fold of the initial amount. With the beginning of the heating period in autumn, the relative humidity in indoor air decreases. This turns out to be the limiting factor in the growth of moulds and mites, Only "relict biotopes" like the mattresses and upholstery still provide conditions to winter and eventually build up a new population in the nest year. This seasonal pattern also applies to the spores of the moulds.
This investigation tried to answer the question of a possible dependency of the occurrence of mites in carpeting, mattresses and upholstery from the type of the heating systems. Radiator and under floor-heating system. Radiator and under floor heating were compared.
21 households were investigated in a randomized trial. 10 dwellings had under floor heating, 11 had radiators. Upholstery and carpeting in the living room, mattresses and carpeting in the bedrooms were inspected for moulds and mites in all 21 dwellings. Under consideration of the above mentioned seasonal pattern specimen were taken in February, may, august, and November of the year 1989. The complete surface of each object was vacuumed for a standardized time of 2 min/sqm. The dust filters were stored below 6 degrees Celsius up to 3 days. The dust filters were weighed for a quantitative analysis. The content of living and dead mites was determined by the floatation method (1) and microscopic inspection. Stout mites with compete bodies (tentacels and legs) were called "living". Mites without these features were called "dead". All determinations were done three-fold.
The mould analysis to determine the colony forming units (CFU) was done by the dilution method (2). This determination was also done three fold. Malt agar plates with an addition of sugar were used as culture matrixes to provide conditions equivalent to those of a relative humidity of 95%. After inoculation the plate were incubated at 25 degrees Celsius for 1 week and then analyzed quantitatively and qualitatively.
The mean number per square meter of living and dead mites (dermatophagoides pternossinus and d. farinae) and of the most common moulds (caldosporium, Aspergillus and Penicillium) were determined. Fig.1 shows the number of living mites in carpeting, In February and November, the number of living and dead mites in radiator heated dwellings is significantly higher than in those with under floor heating. The mean number of living and dead mutes in radiator heated dwellings increases strongly in spring and summer until November. The corresponding numbers in dwellings under floor heatings is significantly lower. There is only a slight increase in the number of living mites in summer. The reducing effect of under floor heating on the growth of mites and moulds can also be found in upholstery and mattresses. In February, we found 624 mites/sqm in upholstery in radiator heating dwellings, while only 53 mites/sqm in dwellings with under floor heating. The number for mattresses are 470 and 34 mites/sqm. In August, we found significantly more living mites in upholstery and mattresses in radiator heated dwellings, while in November the number of dead mites in upholstery in dwellings with under floors heating systems was significantly higher then in radiator heated dwellings.
Under floor heating in an indoor environment provides climatic conditions that reduce the growth of mites and moulds efficiently. Especially in the carpeting of living rooms almost no mites could be found. This effect was not as stronf in the bedrooms, which is most likely related to the fact that these rooms are not as consequently heated. The big heating surface of an under floor heating provides a regular and steady heat ditribution.
In addition, the coldest area of the room is no longer the floor as with radiator heating- and subsequently the carpeting has a lower relative air humidity. Because with an under floor heating the temperature gradient declines from the floor upwards- in contrast to a radiator heating- and subsequently the carpeting has a lower realtive air humidity. Because with an under floor heating the temperature gradient declines from the floor upwards- in contrast to a radiator hearing- this is also of benefit for upholstery and mattresses. This was most evident in our February investigations.
Under floor heating in an indoor environment provides climatic conditions that do not allow moulds to grow in house dust. This is especially true for the Aspergillus group, which lives in a symbiosis with house dust mites (1). Cladosporium, a typical outdoor air mould (3), has its climax in summer and can then be found in similar concentrations in radiator heated and under floor heated dwellings. This mould reaches the indoor space by air through open windows in summer. At first, one is surprised that also in February and may the number of mites in carpeting of radiator heated dwellings, although the heating usually is not used in summer. This is most likely related to the fact that dry dust is much easier vacuumed from the floor, which leaves less nutritive materials to the mites and that for this and other reasons the starting population at the beginning of the generating season is smaller.
The limiting factor "Under floor heating" is an "active preventive factor" in as far as it keeps the allergen producers on a low level throughout the whole year. The consequence is a lower allergen production in dwellings with under floor heating than in radiator heated dwellings. Importantly enough to be mentioned especially for the allergic person is that with the regular and steady heating of the under floor heating there is significantly less airflow and turbulences. Subsequently there are much less allergens in the air. Additionally dust mite resistant mattresses will also assist in eliminating them.
In conclusion, the described results strongly indicate that under floor heating efficiently reduces the growth of mites and moulds in the indoor environment and as a consequence improves the life quality of the allergic person.
(1) Bronswijk J.E.M.H. (1981) "House Dust Biology"
(2) Lustgraaf B. van de (1978) "Ecological Relationship between Microorganisms and house-dust mite"
(3) Wilken-Jensen K., S.Gravesen (1984) "Atlas of Moulds in Europe Causing Respiratory Allergies"