A Discussion of
RELATIVE HUMIDITY LOWER BOUNDARY FOR ASHRAE Standard Project Committee 62,
Ventilation for Acceptable Indoor Air Quality
by
Michael J. Hodgson, MD, MPH, Univ. of Connecticut Health Sciences Center
The questions addressed here are as follows:
1. Is there evidence that individuals voice complaints of more serious or
more frequent eye irritation and discomfort below 30% relative humidity?
2. Is there objective evidence for increased adverse health (eye) effects
occur at relative humidity levels below 30% ?
Conclusions:
There is reasonable evidence that occupant complaints will increase at
relative humidity levels below 30%.
There is no clear evidence that true disease will occur anew; individuals
with a variety of diseases may be at risk for disease exacerbation at levels
below 30%.
Work practices, such as decreased blinking associated with videodisplay
terminal work, and associated physiologic effects, such as increased ocular
surface associated with videodisplay terminal work, may make eye irritation
more severe at given levels of relative humidity because of the greater
tear-water evaporation.
Susceptibility, as evidenced by increased tear-film break-up time, may be an
important predictor of the level of discomfort.
Tear film stability, as measured by tear-film break-up time, is a function
of tear secretion (drug-related, individual factors),tear film stability
(lysozyme, lactoferrin content), and underlying diseases (primary and
secondary conjunctivitis, Sicca syndrome,
DISCUSSION
Standard 55-92 provides a lower bound of 30%, unchanged from prior versions.
No critique of the original literature is provided in that version. Since
then, a number of additional studies have appeared with bearing on the
topic. These include chamber studies and field studies. Sensation of eye
irritation is mediated by irritant receptors, as summarized in work by Cain
and Nilsen, familiar to ASHRAE members. This sensation is a function of
exposure to irritants, tear-film stability across the eye, and underlying
diseases.
Low levels of relative humidity in clean air have been examined in two
chamber studies (Anderson 1973, 1974). These are widely quoted as showing
no effect of relative humidity changes to below 20%.
Anderson et a. (1973) studied 48 medical students in an environmental
chamber. A one-hour control period (at 70% rh) occurred before and after
two hours at 70%, 50%, 30%, and 10%. Although the authors state that no
adverse effects were seen, careful review of the paper suggests that the
group exposed at the lowest level actually did notice dryness, although the
mean vote was between comfortably dry" and "dry" (.1> p > .05). Given the
relatively short exposure period and the relatively small number of subjects
exposed at that level, the findings warrant further study.
Andersen et al. (1974) studied eight students, four in the summer and four
in November, exposed to 9% relative humidity for 72 hours. Individuals did
not describe discomfort, according to the author. The mean vote is at the
boundary between "comfort" and "too dry," suggesting that inadequate power
to detect a difference may be a problem with this study. No further chamber
studies of clean air were identified.
Laviana et al. (1988) were unable to detect differences in eye discomfort
("pain or discomfort") between levels of 30% and 10% in the control eyes of
a study of the effect of humidity on contact lens-associated eye comfort.
They studied 24 students; the report does not mention whether air was clean
(HEPA filtered, etc.)
Nilsson and Andersson (1986) evaluated eye irritation among contact lens
wearers in a cross-sectional field and in an environmental chamber study.
Field work of a groups of subjects with and without eye discomfort suggested
that individuals with complaints were more likely to work at relative
humidities of 30% and below, whereas individuals without complaints worked
in areas with levels of on average around 40%. A small experimental
(chamber) study of 10 subjects suggested that complaints of discomfort
differed both as a consequence of changes in relative humidity and of the
initial tear-film break-up time.
Reinikkainen et al. (1992) conducted a well-known field study of the effect
of humidification on symptoms. The mean relative humidity in control
periods ranged from 18% to 26%, in humidified periods from 26% to 41%. They
demonstrated increased descriptions of dryness and increased numbers of
symptoms among individuals in the non-humidified areas.
Nordstrom et al. (1994) conducted an intervention study by humidifying two
of four similar hospital units during winter months. Relative humidity
changed from a mean of 31% to a mean of 40%. One third as many workers
described dryness of the eyes in the humidified areas (24% as compared to
73%). Frank irritation complaint frequencies did not differ. Sick-building
syndrome symptoms did not differ.
Tsubota and Nakamori (1993) suggest that individuals working on videodisplay
terminal units blinked less frequently and demonstrated a larger exposed
ocular surface at VDU work than while. They felt ocular discomfort in
videodisplay terminal operators might be related to a more rapid evaporation
rate of tears because of, for example, these two factors.
Wyon (1992) constructed a controlled intervention study of nine separate
intervention, including steam humidification, raising the humidity from 25%
to 40%. Symptoms consistent with the sick-building syndrome (using a linear
analog scale) decreased statistically significantly.
Eye irritation, or the sensation of dryness, might result either from low
relative humidity alone or from it together with other pollutants. Then,
other eye irritants would react with the eye surface to cause irritation.
Fiberglass, saturated, non-"polar" homologous series, formaldehyde and other
aldehydes, and mixtures of volatile organic compounds have been described or
studied in isolation. Interactions between these agents and relative
humidity differences do not appear to have been studied. Many of the
studies do not detail how clean the air actually is. Given the current
literature, it does not appear reasonable to lower the bound.
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Hal Levin <hlevin@cruzio.com>
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