Page 290 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

LEED Indoor Envir onmental Quality 253

each person. It may be unique to each person at different stages in his or her life, and as
a person becomes more active, the Mets are increased. A metabolic rate of two to four
times the resting rate is considered to be low to moderate activity. For very intense
activity, the increase may be up to approximately 12 Mets, but usually vigorous exercise
results in approximately 6 Mets. So how do we relate this to the CO levels in a room?
2
When we breathe outside air, we inhale air with oxygen at about 21 percent
(volumetric) and CO at around 380 ppm (0.038 percent volumetric). Outdoor levels do
2
vary regionally due to other anthropogenic and natural activities that produce CO , and
2
these can vary from 300 to more than 500 ppm, but on average the concentration was
approximately 385 ppm in 2007. This average level has been steadily rising over the last
century.
When we exhale air, there is a little less oxygen (and more water) and around
4.5 percent (volumetric) CO , which is then rapidly diluted by the air around it. (Note
2
that when referring to the concentration of a vapor-phase substance, ppm stands for
molar parts per million or the number of molecules of the substance with respect to 1
million molecules of the air mixture. Usually ppm is not a weight-based measure in the
gaseous phase. At typical temperatures and pressures that humans occupy, the gaseous
phase acts approximately like an ideal gas, and molar parts per million can be assumed
to be equivalent to volumetric parts per million. Therefore, the concentration ppm as used
in this text for vapor-phase concentrations is always molar based.) When we are outdoors,
the space is so large that the contribution from our breathing does not noticeably increase
the ambient CO concentration; but when we are indoors, the concentration of the CO is
2 2
elevated in the room since we are adding CO to this set volume. Typically, you can
2
estimate that sedentary activities in a typical space with proper ventilation will raise the
CO concentrations by about 300 to 600 ppm depending on the human density of the
2
room. Better estimates can be made by doing an air concentration model of the indoor
space, but the approximate amounts just stated are a good rule-of-thumb initial estimate
for typical indoor CO level increases.
2
CO is used as a marker compound for several reasons. First, an increase in CO levels
2 2
usually means a decrease in oxygen levels due to consumption by the occupants. Lower
oxygen levels tend to make us more lethargic. Second, if there are higher CO levels, then
2
higher levels of many other air pollutants from the sources of the CO (humans) may be
2
reasonably expected to be in the air. These other pollutants include bacteria and other
pathogens that humans cough or breathe out. In addition, more activity tends to increase
the potential for settled air pollutants to reentrain into the airspace and for other pollutants
to get knocked off of the sources (people) carrying them and into the air. Having monitors
that detect higher levels of CO will allow for either automatic or manual increases in the
2
air exchange rate, so that oxygen levels increase and other pollutant levels decrease if the
air exchange is made with a less polluted air source. An automatic system is referred to as
a demand-controlled ventilation (DCV) system. Usually the outdoor air is less polluted and
contains more oxygen, but air exchanges through filters can also be designed if there are
concerns with the outside air. Usually, at a minimum, mechanical ventilation systems
have some sort of a particulate matter filter.
As mentioned previously, one important parameter in the Indoor Environmental
Quality category is the location of the breathing zone. This is used for both IEQc1 and
IEQc2 for the air quality testing options pertaining to the location of the marker
compound monitors. A schematic of a breathing zone as designated for LEED air quality
purposes can be seen in Fig. 6.0.1. Here the typical breathing zone is a 3-ft-high rectangle
located 3 ft off the floor, and 2 ft from any wall or air handling unit. However, as