It is not only human activities such as
cooking, smoking, and cleaning that affect
the indoor environment. The mere presence
of humans affects the oxidative ability of the
air. Wisthaler and Weschler (6) have shown
that human occupancy can dramatically af-
fect ozone levels, such that concentrations
of this oxidant dropped by half within 30
minutes when two people entered a test
chamber similar in size to a typical indoor
room. At the same time, the concentrations
of various carbonyl compounds increased.
The reactions are so fast that many chemi-
cally reactive oils on human skin may be
transformed into more oxidized molecules
on time scales of tens of minutes (7). Recent
measurements in heavily occupied spaces
such as classrooms illustrate the wealth
of human emissions of not only naturally
formed molecules (such as small organic
acids) but also personal care products (such
as siloxanes) (8, 9). A key uncertainty is the
degree to which indoor environments are
oxidizing. Outdoor light levels drive the pro-
duction of radicals, such as hydroxyl (OH),
which act as atmospheric cleansing agents.
Without high levels of UV light indoors,
what levels of OH will be present? Gómez
Alvarez et al. have reported the detection of
indoor OH radicals, formed from the sun-
light-driven decomposition of nitrous acid
(10); they observed OH concentrations simi-
lar to those that form outdoors.
These findings have placed emphasis on
indoor radical chemistry. It remains unclear, however, whether light is necessarily
involved, or whether dark sources of OH
from the oxidation of gas-phase alkenes by
ozone dominate instead. In particular, terpene oils, which are components of cleaning, fragrances, and cooking materials, are
widely present indoors. Ozonolysis of such
alkenes leads to the formation of highly reactive molecules, the Criegee intermediates.
The latter sometimes decompose to form
OH radicals but in other circumstances
may react with a wide range of other indoor
constituents. In 2017, Berndt et al. reported
the direct mass spectrometric measurement
of gas-phase Criegee intermediates (11); this
work opened up the potential for measuring such species in indoor environments.
Criegee intermediates may also form when
indoor surfaces that are coated with chemically unsaturated skin cells and cooking
oils are exposed to ozone. Although largely
unstudied, such chemistry may form highly
reactive and potentially harmful products,
including peroxides and ozonides, on indoor surfaces (12).
The atmospheric chemistry field has undergone a dramatic transformation in its
understanding of how volatile organic compounds (VOCs) are oxidized (13). Highly
oxidized organic compounds arise via autooxidation mechanisms initiated by either
ozone or radical attack. Reaction with a
single oxidant molecule can form multiple
oxygenated functional groups on an organic
reactant within seconds, changing it from
a volatile gas to a molecule that will condense to form secondary organic aerosol
(SOA) particles (14). Given that levels of
VOCs, such as terpenes, can be much higher
indoors than outdoors, this pathway may
be an important indoor aerosol formation
mechanism. Because of very high levels of
outdoor pollution, VOC concentrations in
industrially developing areas such as China
may be much higher than in European and
North American homes.
The building science research community
has long identified the importance of ventilation for the state of indoor environments.
Open windows expose us to outdoor air,
whereas well-sealed houses are subject to
emissions from furnishings, building materials, chemical reactions, and people and their
activities. Climate change (15) and outdoor
air pollution are leading to efforts to better seal off indoor spaces, slowing down exchange of outdoor air. The purpose may be to
improve air conditioning, build more energy-efficient homes, or prevent the inward migration of outdoor air pollution. As exposure to
indoor environments increases, we need to
know more about the chemical transformations in our living and working spaces, and
the associated impacts on human health. j
REFERENCES AND NOTES
1. S. S. Lim et al ., Lancet 380, 2224 (2012).
2. M. Sleiman et al ., Proc. Natl. Acad. Sci. U.S.A .107, 6576
3. A. Gandolfo, L. Rouyer, H. Wortham, S. Gligorovski,
Appl. Catal. B 209, 429 (2017).
4. C.J. Weschler, W. W.Nazaroff, Indoor Air 22,356(2012).
5. J. P. S. Wong, N. Carslaw, R. Zhao, S. Zhou, J. P. D. Abbatt,
Indoor Air 27, 1082 (2017).
6. A. Wisthaler,C.J. Weschler, Proc. Natl. Acad. Sci. U.S. A.107,
7. S.Zhou,M. W.Forbes, Y.Katrib,J.P.D.Abbatt, Environ. Sci.
Technol. Lett. 3, 170 (2016).
8. S. Liu, S. L. Thompson, H. Stark, P. J. Ziemann, J. L.
Jimenez, Environ. Sci. Technol. 51, 5454 (2017).
9. X. Tang,P.K.Misztal, W. W.Nazaroff,A.H.Goldstein,
Environ. Sci. Technol. 50, 12686 (2016).
10. E. Gómez Alvarez et al. , Proc. Natl. Acad. Sci. U.S. A.110,
11. T. Berndt, H. Herrmann, T. Kurtén, J.Am.Chem.Soc. 139,
12. S.Zhou,M. W.Forbes,J.P.D.Abbatt, Environ. Sci. Technol.
50, 11688 (2016).
13. J. D. Crounse, L. B. Nielsen, S. Jørgensen, H. G. Kjaergaard,
P. O. Wennberg, J. Phys. Chem. Lett. 4, 3513 (2013).
14. M. Ehn et al ., Nature 506, 476 (2014).
15. W. W. Nazaroff, Environ. Res. Lett. 8, 015022 (2013).
J.P.D.A. is supported by the Alfred P. Sloan Foundation. S.G. is
supported by the Chinese Academy of Sciences.
brain stimulation can
by optical triggers
By Neus Feliu,1,2 Erwin Neher,3
Wolfgang J. Parak1,4,5
Neurons can be modified with light- gated ion channels, which cause them to become excited upon illumi- nation (1, 2). This discovery has given rise to the field of optogenetics, with impressive examples such as triggering the beat of a heart with light (3). There
is, however, one technical limitation: Light-gated ion channels are typically stimulated
with blue-green light, which is heavily scattered by tissue. Thus, deep brain stimulation, focused on small regions, is a major
challenge. On page 679 of this issue, Chen et
al. (4) use transgenic mice implanted with
upconverting nanoparticles (NPs) to locally
activate light-gated ion channels and modulate neuronal activity, even deep inside the
brain. This method might eventually lead
the way for clinical applications to optically
control neuronal dysfunctions, such as Parkinson’s disease or even paralysis.
The brain is a gigantic assembly of in-terconnected neurons. A neuron “fires” a
nerve impulse when a sufficient amount of
Na+ ions enter the cell through membrane-bound ion channels. Neuronal Na+ channels
are unusual in that they open even more as
Na+ ions pass through, rendering the inside
of the cell increasingly positively charged
(meaning that these channels are voltage-gated). Thus, a wave of positive charge
spreads along the nerve fiber as a nerve
impulse. Likewise, communication between
1Fachbereich Physik und Chemie and Center for Hybrid
Nanostructures, Universität Hamburg, Hamburg, Germany.
2Department of Laboratory Medicine, Karolinska Institutet,
Stockholm, Sweden. 3Max-Planck-Institut für biophysikalische
Chemie, Göttingen, Germany. 4CIC BiomaGUNE, San
Sebastián, Spain. 5Institute of Nano Biomedicine and
Engineering, Shanghai Jiao Tong University, Shanghai, China.
9 FEBRUARY 2018 • VOL 359 ISSUE 6376 633
Reactive chemicals in an indoor environment
arise from cooking, cleaning, humans, sunlight,
and outdoor pollution.