are also sources of VOCs such as formaldehyde,
carbonyls, and other chemical species like HONO,
which in some cases are worse than their precursors [89, 91]. In addition, it has been shown that
hydroxyl radicals react with the organic binders in
the paint resulting in byproducts that are released
from the paint [109]. A large-scale experiment
conducted as part of the European LIFE+ project
examined the effect of TiO 2 coating on the concrete
inside the Leopold II Tunnel in Brussels; UV lamps
were installed along the length of the tunnel. NO x
removal was measured before and after the TiO 2
coating was applied. In contrast to previous laboratory experiments, no significant reduction of
NO x was observed. An upper limit for NO x
removal at 2% was identified, corresponding to
the uncertainties of the instruments used [92]. In
conclusion, TiO 2 shows some promising air
cleaning properties in laboratory experiments, but
at the same time, there are reasonable concerns,
and TiO 2 products have yet to find widespread
commercial success despite decades of effort.
Plants as Remediation Strategy
In 1989, NASA conducted a study of 12 house
plants selected to tolerate low-light conditions
and determined their ability to remove benzene,
formaldehyde, and trichloroethylene from indoor
air. The study was performed as a chamber experiment at both high (15–20 ppm) and low (<1 ppm)
pollutant concentrations. The plants showed potential for improving IAQ in energy-efficient buildings, by removing pollutants from the air. The rootsoil zone of the plant seemed to be the most efficient at removing pollutants, possibly due to microbial activity in the soil. It was also observed that
when the same plant and potting soil were constantly exposed to a certain pollutant, their capacity
to remove that pollutant increased. Typically,
plants emit trace levels of VOCs and other metabolites. Under the temperature and light conditions
of the experiments, the tested plants emitted negligible amounts of metabolites. As temperature and
light levels increased, the plants’ metabolite emissions would be expected to increase, as well as
their air cleaning properties. Increases in O 2 production and CO 2 removal would be associated with
an increase in the pollutant removal efficiency of
the plant [93]. Other chamber studies show that
selected low-light requiring house plants efficiently
remove formaldehyde [94], xylene, and ammonia
[95], as well as toluene, benzene, trichloroethylene,
octane, and a-pinene [96]. Although chamber studies showed that selected plants have air cleaning
properties, field experiments are needed to confirm
if the plants studied will be capable of significantly
reducing indoor air pollution in a real-world situations. A Sydney field study tested the air cleaning
properties of two common house plants,
Spathiphyllum (peace lily) and Dracaena
deremensis (Janet Craig), in single-occupant
offices. Both naturally ventilated and airconditioned offices were assessed, and the air purifying properties were measured as total VOC
removal. Experiments showed that offices with a
total VOC concentration larger than 100 ppb had a
significant reduction in VOC levels when either
3 or 6 Janet Craig plants were present, and that
3 plants were just as efficient at removing VOCs as
6 plants. This was observed for both airconditioned and naturally ventilated offices. The
study also found that when a mix of Dracaena
deremensis and Spathiphyllum were present,
again either 3 or 6 plants, a significant reduction
of total VOC levels was observed, except when
Spathiphyllum was in flower as it then emits
VOCs. In summary, the total VOC levels were
reduced by 50–75% when the initial VOC concentration was 100 ppb or above, and 3 plants are
sufficient to obtain this reduction under the conditions of the study [97]. Another field study investigated the air cleaning abilities of Dracaena
deremensis (Janet Craig), Dracaena marginata
(marginata), and Spathiphyllum (peace lily) in an
urban classroom in Portugal. Six plants were
placed in a 25-person classroom and air-borne
pollutants were monitored over a 6-week period.
The study found that CO 2 , VOC, carbonyl, PM 10 ,
organic carbon, nitrate, sulfate, ammonia, calcium,
and carbonate levels were significantly reduced
after the introduction of the six plants [98].
Based on the chamber experiments and field
studies, it can be concluded that plants can be
effective air cleaning systems, and a cost-effective
solution for many types of buildings. Since different plant species target different pollutants, it is
154
Indoor Air Quality: Status and Standards
carbonyls, and other chemical species like HONO,
which in some cases are worse than their precursors [89, 91]. In addition, it has been shown that
hydroxyl radicals react with the organic binders in
the paint resulting in byproducts that are released
from the paint [109]. A large-scale experiment
conducted as part of the European LIFE+ project
examined the effect of TiO 2 coating on the concrete
inside the Leopold II Tunnel in Brussels; UV lamps
were installed along the length of the tunnel. NO x
removal was measured before and after the TiO 2
coating was applied. In contrast to previous laboratory experiments, no significant reduction of
NO x was observed. An upper limit for NO x
removal at 2% was identified, corresponding to
the uncertainties of the instruments used [92]. In
conclusion, TiO 2 shows some promising air
cleaning properties in laboratory experiments, but
at the same time, there are reasonable concerns,
and TiO 2 products have yet to find widespread
commercial success despite decades of effort.
Plants as Remediation Strategy
In 1989, NASA conducted a study of 12 house
plants selected to tolerate low-light conditions
and determined their ability to remove benzene,
formaldehyde, and trichloroethylene from indoor
air. The study was performed as a chamber experiment at both high (15–20 ppm) and low (<1 ppm)
pollutant concentrations. The plants showed potential for improving IAQ in energy-efficient buildings, by removing pollutants from the air. The rootsoil zone of the plant seemed to be the most efficient at removing pollutants, possibly due to microbial activity in the soil. It was also observed that
when the same plant and potting soil were constantly exposed to a certain pollutant, their capacity
to remove that pollutant increased. Typically,
plants emit trace levels of VOCs and other metabolites. Under the temperature and light conditions
of the experiments, the tested plants emitted negligible amounts of metabolites. As temperature and
light levels increased, the plants’ metabolite emissions would be expected to increase, as well as
their air cleaning properties. Increases in O 2 production and CO 2 removal would be associated with
an increase in the pollutant removal efficiency of
the plant [93]. Other chamber studies show that
selected low-light requiring house plants efficiently
remove formaldehyde [94], xylene, and ammonia
[95], as well as toluene, benzene, trichloroethylene,
octane, and a-pinene [96]. Although chamber studies showed that selected plants have air cleaning
properties, field experiments are needed to confirm
if the plants studied will be capable of significantly
reducing indoor air pollution in a real-world situations. A Sydney field study tested the air cleaning
properties of two common house plants,
Spathiphyllum (peace lily) and Dracaena
deremensis (Janet Craig), in single-occupant
offices. Both naturally ventilated and airconditioned offices were assessed, and the air purifying properties were measured as total VOC
removal. Experiments showed that offices with a
total VOC concentration larger than 100 ppb had a
significant reduction in VOC levels when either
3 or 6 Janet Craig plants were present, and that
3 plants were just as efficient at removing VOCs as
6 plants. This was observed for both airconditioned and naturally ventilated offices. The
study also found that when a mix of Dracaena
deremensis and Spathiphyllum were present,
again either 3 or 6 plants, a significant reduction
of total VOC levels was observed, except when
Spathiphyllum was in flower as it then emits
VOCs. In summary, the total VOC levels were
reduced by 50–75% when the initial VOC concentration was 100 ppb or above, and 3 plants are
sufficient to obtain this reduction under the conditions of the study [97]. Another field study investigated the air cleaning abilities of Dracaena
deremensis (Janet Craig), Dracaena marginata
(marginata), and Spathiphyllum (peace lily) in an
urban classroom in Portugal. Six plants were
placed in a 25-person classroom and air-borne
pollutants were monitored over a 6-week period.
The study found that CO 2 , VOC, carbonyl, PM 10 ,
organic carbon, nitrate, sulfate, ammonia, calcium,
and carbonate levels were significantly reduced
after the introduction of the six plants [98].
Based on the chamber experiments and field
studies, it can be concluded that plants can be
effective air cleaning systems, and a cost-effective
solution for many types of buildings. Since different plant species target different pollutants, it is
154
Indoor Air Quality: Status and Standards