A review of the health effects of relative humidity in indoor environments suggests that relative humidity can affect the incidence of respiratory infections and allergies. Experimental studies on airborne-transmitted infectious bacteria and viruses have shown that the survival or infectivity of these organisms is minimized by exposure to relative humidities between 40 and 70%. Nine epidemiological studies examined the relationship between the number of respiratory infections or absenteeism and the relative humidity of the office, residence, or school. The incidence of absenteeism or respiratory infections was found to be lower among people working or living in environments with mid-range versus low or high relative humidities. The indoor size of allergenic mite and fungal populations is directly dependent upon the relative humidity. Mite populations are minimized when the relative humidity is below 50% and reach a maximum size at 80% relative humidity. Most species of fungi cannot grow unless the relative humidity exceeds 60%. Relative humidity also affects the rate of offgassing of formaldehyde from indoor building materials, the rate of formation of acids and salts from sulfur and nitrogen dioxide, and the rate of formation of ozone. The influence of relative humidity on the abundance of allergens, pathogens, and noxious chemicals suggests that indoor relative humidity levels should be considered as a factor of indoor air quality. The majority of adverse health effects caused bLy relative humidity would be minimized by maintaining indoor levels between 40 and 60%. This would require humidification during winter in areas with cold winter climates. Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens.

A review of the health effects of relative humidity in indoor environments suggests that relative humidity can affect the incidence of respiratory infections and allergies. Experimental studies on airborne-transmitted infectious bacteria and viruses have shown that the survival or infectivity of these organisms is minimized by exposure to relative humidities between 40 and 70%. Nine epidemiological studies examined the relationship between the number of respiratory infections or absenteeism and the relative humidity of the office, residence, or school. The incidence of absenteeism or respiratory infections was found to be lower among people working or living in environments with mid-range versus low or high relative humidities. The indoor size of allergenic mite and fungal populations is directly dependent upon the relative humidity. Mite populations are minimized when the relative humidity is below 50% and reach a maximum size at 80% relative humidity. Most species of fungi cannot grow unless the relative humidity exceeds 60%. Relative humidity also affects the rate of offgassing of formaldehyde from indoor building materials, the rate of formation of acids and salts from sulfur and nitrogen dioxide, and the rate of formation of ozone. The influence of relative humidity on the abundance of allergens, pathogens, and noxious chemicals suggests that indoor relative humidity levels should be considered as a factor of indoor air quality. The majority of adverse health effects caused by relative humidity would be minimized by maintaining indoor levels between 40 and 60%. This would require humidification during winter in areas with cold winter climates. Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens.

One obstacle in applying the very practical techniques of automatic calculation of multifield dose distributions has been the lack of a simple yet exact and workable mathematical description of the per cent depth dose distribution within a single beam. Past work by ourselves (Sterling, Perry and Bahr, 1961; Sterling, Perry and Weinkham, 1963b) and others (Tsien, 1955;1958) has been based on procedures in which the isodose curves, obtained by actual measurements, were digitised by hand for different field sizes and stored on cards or tape. Simple convergent dose distributions were then obtained by systematic summing procedures. For non-convergent and other more complex multifield plans per cent depth dose values were obtained by interpolative techniques (Sterling et al., 1963a, b). While the interpolation procedures developed by us gave relatively accurate results even for complex treatment plans, they did not lend themselves to rotations or more sophisticated optimisations. Finally, despite their simplicity, all hand digitising methods limit calculations to those portal sizes for which digitised values exist.It was our aim to find a simple yet reasonably accurate mathematical expression for the depth dose distribution isodose curves of a beam coming through a portal of any length and width. The work was guided by two suppositions which turned out to be justified:(1) there should exist a simple relationship between the per cent depth doses at points away from the central axis and those on the central axis at the same depth; and (2) that the decrement of dose along the major axis may be expressed as a function of field configuration and depth below the surface. The mathematical expressions developed in the following pages apply to 60 Co ionising radiation sources at 80 cm SSD. Similar work is under way now for other sources of energy. Per cent dose on the major axis as a function of portal size and depth below surfacePfalzner (1960) has recently pointed out that tumour-air ratios for 60 Co radiation plotted on full logarithmic paper against field area, with depth as parameter, are straight lines. The fact that the relation is linear in logarithms is actually beside the point. It is important that axial depth dose may be expressed as some function of depth and field size. A similar relation should hold between per cent depth dose and field size. However, while Pfalzner considered the relation between field area and dose satisfactory, experimental evidence indicates that fields of equal areas but different shapes give rise to different per cent depth doses on the central axis. But if field area is corrected for perimeter of the field, the relation between per cent depth dose and (field area/field perimeter) is almost perfect. It is possible, therefore, to express per cent depth dose on the central axis for any depth as a function of (area/perimeter) of the field such that log C t =hi + m t log (AjP) (1) whereCi is the per cent depth dose on the central axis of the field at distance T>% from the skin. AjP is the r...

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