Theoretical HEPA Efficiency vs. Actual System Efficiency Due
to the high efficiency, reliability and proven track-record, HEPA
technology has become the industry standard for particulate filtration
in critical environments, such as semi-conductor cleanrooms and
hospital operating rooms. Recognizing the great marketing potential
of the term "HEPA", many manufacturers are using it to project a
high- performance image onto their room air purifiers. What most
HEPA-based air purifiers manufacturers conveniently omit to tell
the consumer, is that their air purifiers provide nowhere near the
performance level of professional HEPA filter systems used in hospitals
consumers are misled about an air purifier's efficiency in removing
pollutants. Statements like "effectively removes 99% of all airborne
allergens" or "efficiently scrubs the room free of air pollutants",
lead consumers to believe that these air purifiers remove virtually
all of the impurities from the air in a normal indoor environment.
about HEPA-based air purifiers often state 99.97% filtration efficiency.
In most cases, this is also not true. The actual efficiency, for
particles of 0.3 microns or larger, of many HEPA-based air purifiers
sold today is below 80%. The "99.97%" refers to, in most cases,
the theoretical efficiency rating of the filter paper that is used
in the air cleaning device at 0.3 microns or larger.
performance gap between HEPA-based room air purifiers and professional
HEPA filter systems is mainly due to cutting corners in mass-production
and profit maximization. There are a number of reasons why most
HEPA-based room air purifiers do not achieve actual HEPA performance:
HEPA-rated filter media is not used. 2. The HEPA filter media gets
damaged during the pleating process (HEPA media breaks easily).
3. There is leakage between the pleated HEPA filter pack and the
filter frame. 4. There is leakage between the HEPA filter frame
and the air purifier housing.
very few manufacturers state the actual overall efficiency of their
device, and even fewer guarantee and certify their air purifiers.
Overstating Actual Air Delivery Rates Even the most efficient room
air purifier in the world would not impact a room's air quality
level, if it were not able to pass enough air through its filters.
The amount of air that an air purifier is able to process (air flow
rate) is usually expressed in cubic feet per minute (cfm). Consumers
are often misled by exaggerated or false claims by manufacturers
regarding air flow rates.
common trade practice is to state the free-flow air handling capacity
of the fan motor (i.e. without filters installed), rather than the
actual air flow rate (i.e. with all filters installed).
manufacturers don't even make this vital air purifier statistic
available to consumers. Instead, many state a suitable room size,
without mentioning how many air changes per hour the air purifier
would manage to produce in that size room.
very few manufacturers state the actual airflow rate of their device
(with all filters installed), and even fewer guarantee and certify
their air purifier's air flow rate.
ULPA is NOT Better than HEPA In an effort to differentiate air purifiers
from the mass of HEPA air purifiers on the market, some manufacturers
have introduced air purifiers, which supposedly use ULPA technology.
ULPA stands for Ultra Low Penetration Air and is used in modern
cleanrooms. The difference between HEPA and ULPA is the efficiency
rating of the filter media. Typical HEPA filter media has an efficiency
of 99.97% at 0.3 microns, whereas ULPA media has an efficiency rating
in excess of 99.999% at 0.12 microns.
the higher efficiency at smaller particle size, manufacturers of
ULPA-based air purifiers claim that these devices are superior to
HEPA-based air purifiers. Unfortunately, this is a perfect example
where the search for a better sales pitch has resulted in an inferior
product, since the reverse tends to be the case. All things being
equal, an air purifier that uses ULPA filter media will be less
effective in reducing the particle concentration in a typical room,
than the same air purifier equipped with HEPA filter media. And
as with many HEPA-based air purifiers, many ULPA-based air purifiers
come nowhere close to actually delivering 99.999% particle-free
ULPA filter media has the potential to remove more particles than
HEPA filter media, that advantage is lost in room air purifiers,
due to the reduction of air flow caused by the denser ULPA filter
media. ULPA filter media typically allow 20-50% less air to pass
than HEPA filter media, resulting in fewer air changes per hour
in a given room.
Ineffective Gas Phase Filtration In many cases room air purifiers
are purchased to deal with gaseous contaminants and odors. So it
comes as no surprise that most manufacturers claim to use some technology
to reduce gases and odors.
most common technology adopted by air purifier manufacturers for
the removal of gaseous pollutants is activated carbon. While activated
carbon in its granular form is unquestionably effective for the
removal of many gaseous contaminants, most room air purifiers on
the market today use carbon fiber pads, which are only impregnated
with activated carbon dust. These filter pads contain only a few
ounces of actual activated carbon and as a result, are essentially
ineffective for the removal of gases and odors.
manufacturers claim that their activated carbon filter eliminates
all gaseous pollutants and odors, a feat that is scientifically
impossible. Specific gaseous contaminants need specific gas phase
filtration technology. Activated carbon does not efficiently adsorb
low molecular weight gases. For this reason, special sorbents are
needed to effectively deal with pollutants, such as formaldehyde,
hydrogen sulfide and ammonia.
room air purifiers that use granular activated carbon also use zeolite.
Zeolite is a natural mineral, whose pore structure is supposed to
be better suited for the removal for gaseous compounds like formaldehyde
and ammonia. In fact, there is no reliable scientific evidence to
show that zeolite is able to remove any gaseous compound better
than specialty impregnated carbons or impregnated alumina. So why
is Zeolite used? Zeolite is an inexpensive "filler" that is less
expensive than activated carbon. Secondly, the concept of using
two different substances to control gases and odors sounds quite
promising to the unsuspecting customer.
Sacrificing Long-Term Performance While most tests and evaluations
of air purifiers assess only air purifier performance during its
first few hours of usage, the real test for air cleaning effectiveness
is long term performance. Unfortunately, most manufacturers cut
costs on features that would enhance long-term performance.
standard industry practice is the use of ineffective pre- filtration.
In HEPA based air purifiers, it causes the HEPA filter to become
clogged at a rapid rate. This in turn reduces the air flow rate,
thus resulting in fewer air changes per hour in a given room. This
is a particularly serious issue, since some manufacturers inform
their consumers that their HEPA filters will only need replacing
every 3 to 5 years.
pre-filtration, also, causes air purifiers with substantial granular
activated carbon filters to quickly lose their gas phase adsorption
potential. Dust particles - that should be trapped by pre-filtration
- clog the miniscule pores of the activated carbon, and destroy
its holding capacity to adsorb gases.
reason for poor long term performance of air purifiers is that some
air cleaning technologies suffer drastic reductions in air cleaning
efficiency without regular maintenance or frequent filter change.
purifiers with electrostatic precipitator technology experience
drastic efficiency reductions as the collector plates become covered
with particles. Air purifiers that use electrostatically charged
fibers (electrete) also rapidly loose filter efficiency with particulate
loading, especially in the presence of tobacco smoke.
Frequent and Expensive Filter Replacement While the initial purchase
cost for many room air purifiers is relatively low, the cost of
replacement filters can be substantial. Here are some reasons why
many air cleaners require frequent and expensive filter replacement:
of effective pre-filters that protect activated carbon and HEPA
filters from premature clogging.
· Use of small filter cartridges
with low holding capacity for pollutants.
· Filter replacement instructions
that base filter replacement intervals on time passed (e.g. every
6 months) rather than actual usage and degree of air pollution.
· Combining several different filter stages in one filter cartridge,
thus forcing the user to replace all filters at once, even if only
one filter stage is used up.
Trying to be All Things to All People While many air purifier manufacturers
offer several models, these models often vary only in size, rather
than air cleaning technology. And since manufacturers like to have
their product appeal to as many potential customers as possible,
they sell one and the same unit as the ideal solution for pet allergens,
pollen, mold spores, microorganisms, tobacco smoke, odors, traffic
fumes and chemicals, etc. In fact, some manufacturers claim that
by virtue of using a multitude of filter stages their air purifier
is more effective. Unfortunately, the effectiveness of the air purifier
suffers, because more filter stages mean higher air resistance and
lower air delivery. Air purifiers that only contain filter stages
matched to the user's requirements, offer superior filtration efficiency
and air flow.
Industry organizations, like AHAM, have set out to establish some
reference point for air purifier performance, they have not succeeded
to make comparison shopping for air purifiers a simple task. AHAM's
testing protocols do not evaluate actual particle retention, gas
phase filtration or long-term performance of air purifiers.
what are the lessons when evaluating air purifiers? Don't believe
every claim made by manufacturers. Realize that a $80 air purifier
will have serious limitations. Ask manufacturers to substantiate
their claims with independent evidence. Check the underlying technology
and investigate whether it has been properly implemented. Use tools
that allow objective evaluation of air cleaning performance, such
as laser particle counters, to check manufacturer's particulate
efficiency claims and evaluate actual particle reduction.
claims regarding air purifiers and air filters have in the past
led to decisions and orders of the Federal Trade Commission against
manufacturers, and are likely to do so in the future unless manufacturers
practice better self-restraint and self-regulation. Participating
in sound business practices and making scientifically proven claims
will restore lost faith in room air purifiers, allowing consumers
to make informed purchase decisions - based not only on price but,
also, on actual performance. http://www.stores.yahoo.com/allergybuyersclub/iqair.html