low wind speeds and cold outside temperatures.
A mixed-mode system can be operated manually
or automatically, so when the set values for temperature, humidity, and CO 2 levels are not maintained
by natural ventilation, the system will shift to
mechanical ventilation, where an inlet fan introduces outdoor air and an exhaust fan sucks out
indoor air. In colder climates, a heat recovery system
can be used. Cold outdoor air is heated by the
exiting indoor air, before entering the room
[84]. When outdoor conditions are favorable, the
system will change back to natural ventilation.
Mixed-mode ventilation favors natural ventilation,
when possible, for optimal energy efficiency.
Activated Carbon
Activated carbon (AC), also called activated charcoal, is a form of carbon processed to have an
extremely high surface area. Its high surface-tomass and surface-to-volume ratio, combined with
low price and high affinity for many kinds of pollution, have made it a valuable material in pollution
control systems. The surface chemistry is based on
graphene, but modified depending on impurities and
modifications. AC is produced from a carbonaceous
material which could be coal, wood, or some other
plant fiber. The material is heated to drive off volatile components, leaving behind a solid with a network of pores. The porous structure consists of long
networks of channels which are termed micro(<2 nm), meso- (2–50 nm), and macropores
(>50 nm). Because of this porous structure, AC
has a specific surface area in the range of
500–3000 m
2
/g, which is crucial for the performance of AC [85]. Many types of pollution can be
trapped by physisorption or chemisorption.
The surface of AC contains functional groups
such as epoxy bridges and carboxyl, hydroxyl, and
carbonyl moieties. These groups change the chemical properties of the surface and can be used to
change the hydrophilicity of the surface and the
affinity for specific types of species. The process of
adsorption, including surface chemistry, is shown
in Fig. 6. Even though AC has been widely used
for many years, improving the adsorption properties is an active area of research. One example is the
addition of a basic nitrogen functionality to the
surface to improve CO 2 uptake [86].
Photocatalysis
A potential drawback of using adsorbents is that
they do not break down pollutants, but instead the
pollutants remain on the absorbent, which has a
limited capacity. Adsorbent capacities may be in
the range of 1–10% of the mass of the filter material itself, and eventually they reach capacity and
must be changed. Photocatalysts, in contrast, can
be regenerated continuously using light.
Some photocatalysts can be driven by sunlight,
and therefore do not necessarily rely on an external source of energy, typically electricity. Furthermore, they promise to transform hazardous VOCs
into H 2 O and CO 2 , unlike conventional filter systems, which rely on trapping VOCs. However,
research has shown that many photocatalysts do
not live up to this goal and leave many compounds only partially oxidized; they convert pollution into byproducts that may smell worse and
be more toxic than the precursors [87].
TiO 2 , in particular, has received significant
attention from researchers because of its photocatalytic activity and low cost. TiO 2 has been used
in a wide range of applications including purification of wastewater and air. For purification of air,
the material has been placed on window glass and
pavement or incorporated directly into materials
like paint and concrete [88]. The activity of TiO 2
is greatly improved when using nanosized particles as opposed to microsized particles. This is
due to the increased surface area, which greatly
increases the photocatalytic activity, as there are
more sites available for molecules to adsorb and
reactions to occur. However, the use of nanoparticles can pose a threat to human health. One
study showed that significant amounts of nanoparticles are released into the environment due to
aging of photocatalytic paints [89]. Nanoparticles
are very toxic to humans and the environment in
general and there is a need for improving nanosized TiO 2 materials.
The first step is transport of the pollution to the
catalytic surface (see discussion of deposition
onto a surface above) followed by adsorption.
Different molecules will spend different amounts
of time on the surface depending on how tightly
they are bound. Photocatalysts have a limited
ability to degrade volatile pollutants, simply
152
Indoor Air Quality: Status and Standards
A mixed-mode system can be operated manually
or automatically, so when the set values for temperature, humidity, and CO 2 levels are not maintained
by natural ventilation, the system will shift to
mechanical ventilation, where an inlet fan introduces outdoor air and an exhaust fan sucks out
indoor air. In colder climates, a heat recovery system
can be used. Cold outdoor air is heated by the
exiting indoor air, before entering the room
[84]. When outdoor conditions are favorable, the
system will change back to natural ventilation.
Mixed-mode ventilation favors natural ventilation,
when possible, for optimal energy efficiency.
Activated Carbon
Activated carbon (AC), also called activated charcoal, is a form of carbon processed to have an
extremely high surface area. Its high surface-tomass and surface-to-volume ratio, combined with
low price and high affinity for many kinds of pollution, have made it a valuable material in pollution
control systems. The surface chemistry is based on
graphene, but modified depending on impurities and
modifications. AC is produced from a carbonaceous
material which could be coal, wood, or some other
plant fiber. The material is heated to drive off volatile components, leaving behind a solid with a network of pores. The porous structure consists of long
networks of channels which are termed micro(<2 nm), meso- (2–50 nm), and macropores
(>50 nm). Because of this porous structure, AC
has a specific surface area in the range of
500–3000 m
2
/g, which is crucial for the performance of AC [85]. Many types of pollution can be
trapped by physisorption or chemisorption.
The surface of AC contains functional groups
such as epoxy bridges and carboxyl, hydroxyl, and
carbonyl moieties. These groups change the chemical properties of the surface and can be used to
change the hydrophilicity of the surface and the
affinity for specific types of species. The process of
adsorption, including surface chemistry, is shown
in Fig. 6. Even though AC has been widely used
for many years, improving the adsorption properties is an active area of research. One example is the
addition of a basic nitrogen functionality to the
surface to improve CO 2 uptake [86].
Photocatalysis
A potential drawback of using adsorbents is that
they do not break down pollutants, but instead the
pollutants remain on the absorbent, which has a
limited capacity. Adsorbent capacities may be in
the range of 1–10% of the mass of the filter material itself, and eventually they reach capacity and
must be changed. Photocatalysts, in contrast, can
be regenerated continuously using light.
Some photocatalysts can be driven by sunlight,
and therefore do not necessarily rely on an external source of energy, typically electricity. Furthermore, they promise to transform hazardous VOCs
into H 2 O and CO 2 , unlike conventional filter systems, which rely on trapping VOCs. However,
research has shown that many photocatalysts do
not live up to this goal and leave many compounds only partially oxidized; they convert pollution into byproducts that may smell worse and
be more toxic than the precursors [87].
TiO 2 , in particular, has received significant
attention from researchers because of its photocatalytic activity and low cost. TiO 2 has been used
in a wide range of applications including purification of wastewater and air. For purification of air,
the material has been placed on window glass and
pavement or incorporated directly into materials
like paint and concrete [88]. The activity of TiO 2
is greatly improved when using nanosized particles as opposed to microsized particles. This is
due to the increased surface area, which greatly
increases the photocatalytic activity, as there are
more sites available for molecules to adsorb and
reactions to occur. However, the use of nanoparticles can pose a threat to human health. One
study showed that significant amounts of nanoparticles are released into the environment due to
aging of photocatalytic paints [89]. Nanoparticles
are very toxic to humans and the environment in
general and there is a need for improving nanosized TiO 2 materials.
The first step is transport of the pollution to the
catalytic surface (see discussion of deposition
onto a surface above) followed by adsorption.
Different molecules will spend different amounts
of time on the surface depending on how tightly
they are bound. Photocatalysts have a limited
ability to degrade volatile pollutants, simply
152
Indoor Air Quality: Status and Standards