To a person with a pollen allergy, an 18-acre ragweed
field sounds like a sneezy, red-eyed zone
of misery. But to two environmental engineering researchers
at Johns Hopkins, the parcel presented a
rare and valuable opportunity to learn how the troublesome
weeds grow, reproduce and scatter their
pollen under varying weather conditions.
Their findings, gathered with a mix of high-tech and
low-tech tools, could lead to better ways
to track the pollen's travel and control the pesky plant's
spread, discoveries that could aid the 15
million people with ragweed allergies in the United States
and Canada alone. And although the plant is
native to North America, the nuisance appears to be
spreading. Researchers say the plant has invaded
China, Japan and parts of Australia, and is now moving
rapidly across Europe as well. To address this
problem, the Johns Hopkins team is using data from the
18-acre field to develop a computer model of
ragweed pollen behavior. The model also could someday help
to predict the spread of bioengineered
corn pollen before it contaminates natural crops.
Under the guidance of faculty advisers, the ragweed
research is being carried out by Mike
Martin and Marcelo Chamecki, doctoral students in the
Whiting School's Department of
and Environmental Engineering. At the onset of ragweed
pollen season last year, the students set out
to find a real-world lab site in which to collect data.
Just outside of Washington, D.C., they stumbled
upon a large tract of vacant land that was covered by a
dense growth of the plant. With the property
owner's permission, they set up camera and computer
equipment, meteorological gauges and pollen-
collecting instruments to gather information about ragweed.
They have spent the past year analyzing
these data and hope to publish some of their findings soon
in a scientific journal. The research will
also serve as the foundation for their doctoral theses.
Although neither of them is allergic to ragweed, both
students know how easily it can trigger a
response among those who are. "Concentrations of fewer than
10 pollen grains per cubic meter can
cause an allergic reaction in people who are sensitive to
ragweed," Martin said. "During our field
research, we found concentrations of 10,000 grains per
cubic meter in the air above the plants. My
clothes were stained yellow with pollen."
Although ragweed is not new to North America,
scientists have determined that it has spread
significantly throughout the continent since the arrival of
European settlers. The newcomers cleared
shady forest areas to create farmland, enabling ragweed to
flourish in the sunny open spaces.
"I'm trying to develop a detailed description of the
recent evolution of ragweed populations,"
Martin said. "I want to know how the plant's structure and
behavior have influenced its success as an
invasive weed. If we can understand how ragweed was adapted
to its prehistoric environment, we may
find better ways to control its harmful effects in the
present day by predicting when the pollen will
be released and where it will end up."
His fellow researcher has somewhat different aims. "My
main interest is learning how the wind
spreads the pollen under different turbulence and
temperature conditions," Chamecki said. "I want to
use the data from our field experiments to develop and
calibrate a computer model. This model could
be used to predict how pollen grains are likely to spread
under different topographic and atmospheric
conditions. If the computer model works for ragweed, it
should also work for other types of pollen and
other tiny airborne particles and organisms like bacteria,
soot and even snowflakes."
In the immense ragweed field, the researchers gathered
data by first marking off 25 randomly
selected study sites, each measuring one square meter.
Based on a survey of the plants in each of
these samples, they concluded that each square meter of the
field contained about 90 ragweed plants,
including seedlings. When they considered that one small
plant is capable of releasing 1 billion grains of
pollen per season, the researchers realized that this
single field probably caused a lot of suffering
for allergic people living nearby.
The students knew that ragweed reproduces in the late
summer and early fall, when male
flowers release pollen to fertilize female portions of the
plant, which release seeds. To study this
process, they aimed a video camera at male flowers, using a
close-up lens. The camera captured
microscopic images of pollen being released in clumps of
about 500 grains each. The student
researchers later were able to document the way such clumps
begin to fall apart and disperse as they
move through the air.
To find out how the airborne pollen concentration
changes as the clumps move away from the
plants, Martin and Chamecki set up an 18-foot-tall pole
equipped with six pollen samplers mounted at
different heights. Each sampler spun a rod coated with
sticky silicone to capture pollen clumps moving
through the air. The students have been able to count and
study the pollen grains by placing the
samples under a microscope.
To document weather conditions at the time of each
sampling, the students assembled a
meteorological tower measuring six meters tall. The tower
was equipped with instruments to measure
solar radiation, air temperature, humidity, wind direction,
wind speed and turbulence. This information
was collected by a datalogger device and stored on a memory
card that could be uploaded regularly to
a laptop computer. The tower's devices were powered by a
car battery recharged by solar cells.
The students left the ragweed field with two weeks'
worth of pollen behavior data that they
are continuing to organize and analyze, including 3,000
pollen samples and many gigabytes of computer
files. Johns Hopkins faculty members involved in the
project are Grace Brush and Marc Parlange,
professors in the Department of Geography and Environmental
Engineering, and Charles Meneveau, a
professor in the Department of Mechanical Engineering.
Brush said Martin and Chamecki's work is
important because "it combines the biological
characteristics of the plant and the mechanisms of
pollen release and transport in the atmosphere, providing
insight into the adaptation of the species to
The research was supported in part by funding from the
National Science Foundation.