Re: GBlist: Relative Humidity

Hal Levin (hlevin@cruzio.com)
Mon, 03 Mar 1997 17:17:08 -0800

At 06:33 PM 3/3/97 -0500, Greg Thomas wrote:
>There are a variety of parameters to control ventilation: CFM per person,
>Air Changes, CO2, and RH. If you had to pick one, which one would you pick
>and why?
>
>Based on the dust mite discussion, I wonder if controlling for RH might be
>the healthiest control parameter, given that you stay within certain bounds
>of minimum cfm per person and maximum cfm (energy efficiency). A range of
>RH levels would allow the occupant to adjust to different health needs at
>different times.
>
>Greg Thomas

What's "healthiest" is a function of what you are controlling for and what
sources of physiological stress are present. This is not so easy to determine.

I do not accept the question "which one would you pick and why?" That would
be like asking a driver if they had to pick whether to control highway
traffic conditions, road surface conditions, travel speed, condition of the
car's brakes, condition of the car's tires, or condition (alertness,
sobriety, physical ability, etc.) of the driver her or himself, which one
would you pick. Problems with any one of those can be sufficient to cause a
serious accident causing personal injury and property damage not only to the
driver of concern but also to other drivers. Picking one without reference
to the others is simply not sufficient. They are all components of highway
safety or contributors to its absence.

If you are concerned about dust mites, then control for dust mites. RH seems
to be critical, but if it gets cold enough (too cold for human comfort) you
probably won't find any dust mites either. You can control RH while making
the indoor environment uncomfortable, unhealthy, or even lethal if you
ignore the other factors. So, specify ranges of acceptable conditions FOR A
GIVEN PURPOSE AND OCCUPANCY.
1. Air changes is appropriate for non-variable sources but must be
adjusted for variable ones.
2. CFM per person is appropriate for occupant driven pollution loads
(moisture or other), but not sufficient without knowing the metabolic rate
or activity level of the occupants.
3. RH is, in fact, an integration of air temperature, absolute moisture
generation rate from all sources including release from sinks, moisture
entering in ventilation air, and moisture removal rate by ventillation and
by deposition or absorption by sinks.
4. CO2 is only a very crude way to assess the adequacy of ventilation
for removal of human source bioeffluents (including, of course, exhaled
water vapor) and, perhaps, heat.

Hal
Hal Levin <hlevin@cruzio.com>

__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________