快猫短视频

PERCHED ON the roof of an ageing laboratory, the green metal device hums
quietly. With each gust, it turns to face into the wind, all the while breathing
in air through a narrow slit at roughly the same rate as a human. Suddenly
someone sneezes. 鈥淭hey鈥檙e about, all right,鈥� remarks the machine鈥檚 custodian,
for this rooftop robot in Cambridge is part of Britain鈥檚 contribution to an
international surveillance network. Its mission: to monitor the movements of
pollen and all the rest of the microscopic life that sails through our
atmosphere.

Already this network has detected telltale signs of climate change, and
picked up worrying hints of new allergy epidemics poised to erupt. It also
produces pollen counts to warn hay-fever sufferers when to stay indoors or make
for the coast. Aerobiologists, as the scientists who operate the network call
themselves, are even contributing to the debate about whether it is safe to grow
genetically modified crops.

Each year, as the world鈥檚 flora goes sex-mad, airborne pollen鈥攖he
flowering plant鈥檚 equivalent of semen鈥攁long with spores from fungi and
non-flowering plants unleash an epidemic of snuffles, sneezes, itchy eyes, or
worse. Landing in the lush mucous membrane of a human nose, pollen grains behave
as if they had found a moist female flower part, and unleash their surface
proteins. In susceptible individuals these alien molecules set off an immune
cascade centred on antibodies belonging to the class known as immunoglobulin E.
The main aim in life of these antibodies is to help mast cells and eosinophils
take action against parasites that penetrate the gut wall. The eosinophils
release poisons to kill the parasites, and the mast cells squirt out histamine
to trigger a flood of mucus to wash them away. When IgE mistakes pollen for a
dangerous parasite you get hay fever or 鈥渁llergic rhinitis鈥濃�攅ssentially a
snuffly nose.

But not all pollen is to blame. Plants that spread their sex cells on the
wind, rather than relying on insects to do the job, trigger most allergies.
Allergenic plants also have to be common, so that the immune systems of
susceptible individuals can be sensitised through repeated exposure. In Greece
and Spain, olive trees are a prime source of allergenic pollen. In North America
ragweed is arguably the worst culprit. In Japan, it鈥檚 red cedar, while in
Scandinavia and parts of central Europe, silver birch is the one to watch.
Everywhere, grass pollen is a scourge.

A trained eye, assisted by a microscope, can tell these particles apart by
their size and surface pores, pits and furrows. These crevices hide the
quick-release proteins that act as chemical passwords for pollen grains seeking
entry into a female flower鈥檚 inner sanctum. A handful of specialists round the
world can even name the tiny, often colourless spores of fungi and plants such
as bracken, which can spread plant diseases or trigger asthmatic attacks.

Hay fever afflicts a quarter of the population in Britain, a third in the US
and perhaps as much as 40 per cent in Australia, says Martin Stern, a clinical
immunologist at the Leicester Royal Infirmary. The condition was apparently
unknown before 1800 and was still relatively uncommon earlier this century, even
though pollen levels are probably no higher now than they were then. Modern
living conditions, the decline in childhood infections, air pollution鈥攁ny
or all of these could be spurs to the growing epidemic.

The prevalence of hay fever has made forecasting pollen levels a priority.
Each morning in spring and summer, researchers in 33 stations around Britain
count the grass pollen grains caught the previous day by the circle of greased
tape in their traps, and send the data to the European Aeroallergy Network鈥檚
database in Vienna. When grass pollen counts are expected to exceed 150 grains
per cubic metre of air, Britain鈥檚 hay-fever sufferers are advised to stay
indoors. More than a dozen specialist centres also monitor tree and weed pollen,
as well as fungal spores.

Pollen traps

In the US, the National Allergy Bureau runs a pollen counting network that
provides local readings three times each week from stations across the country.
Any tree pollen reading over 1500 is considered 鈥渧ery high鈥�, so earlier this
year when these counts exceeded an exceptional 6000 forecasters urged allergic
mid-Westerners to take cover. And in Australia, to forewarn allergic athletes,
routine monitoring is now supplemented by extra pollen traps at the three sites
chosen for next year鈥檚 Olympic Games.

Forecasting the day-to-day ups and downs of the grass pollen season is a
tricky business. To predict pollen loads even a day or two in advance,
aerobiologists have to rely on the weather forecasts for sunshine, humidity,
wind speed and the like. The warmer, drier and windier it is, the higher the
pollen count is likely to be. For longer-term seasonal forecasts, years of
hard-won empirical data are called into play.

Even within Britain, the importance of each meteorological factor varies from
place to place. 鈥淩ainfall is more important in predicting seasonal severity in
Cardiff, while temperature is more important in Derby,鈥� says Jean Emberlin,
director of the National Pollen Research Unit at University College
Worcester.

All this activity has allowed the production of pollen 鈥渃alendars鈥�, from
which sufferers can work out which plants they are allergic to. In North
America, snuffles in spring usually signal an allergy to tree pollen, for
instance, while autumnal rhinitis points the finger at ragweed.

But sometimes attacks come out of the blue, leaving the experts mystified. In
the 1980s, a run of unexplained asthma outbreaks linked to summer thunderstorms
were reported in Australia and Britain. Then on the night of 24 June 1994, the
biggest epidemic to date struck central and southern England. Hospital accident
and emergency departments were packed with thousands of people, mainly in their
20s and 30s. Most had a history of hay fever but had never suffered an asthmatic
attack before. For a time, emergency services were stretched. Luckily, the
epidemic was over in a couple of days. But what had caused it?

After much detective work, aerobiologists and doctors have pieced together an
answer. At first, researchers suspected some malign collusion between pollen and
air pollution. But records showed that in London, where the epidemic was worst,
air quality was not unusually bad. Pollen counts, by contrast, revealed that
grass pollen was at its peak shortly after the storm hit. That seemed to
implicate pollen鈥攅xcept that pollen grains are too large to travel deep
into the airways and trigger asthmatic attacks.

Fungal spores were in the clear, thanks to research by Ursula Allitt at the
University of Cambridge. These spores are small enough to penetrate the smallest
airways, but those known to cause asthma were present in only low concentration
in pollen traps at nine sites hit by the storm鈥攑robably because of dry
weather in the previous month鈥攁nd so could not have been the culprit.

The mystery was eventually solved thanks to earlier work by a team of
Australian researchers led by the late Bruce Knox at the School of Botany at the
University of Melbourne. In a study of an asthma outbreak in Melbourne in 1989,
Knox and his team discovered that grass pollen grains caught in a thunderstorm
became saturated with water and eventually broke up, releasing tiny starch
granules bearing allergenic proteins similar to those found on whole pollen.
These granules were small enough to be drawn deep into the lungs, provoking
asthmatic attacks in people susceptible to hay fever. Blood samples taken at
London hospitals from 16 people caught up in the 1994 epidemic fit that story.
Most had very high levels of IgE antibodies against grass pollen, but showed no
antibody reaction against fungal spores.

Besides keeping tabs on allergenic pollens and spores, aerobiologists can
also provide early warning of changing patterns of the world鈥檚 flora. In
Britain, records of airborne pollen stretch back four decades鈥攖he longest
continuous data set of its kind anywhere in the world. Data from Cardiff, London
and Derby鈥攖he three oldest monitoring sites鈥攔eveal that the April
start of the birch pollen season is about five days earlier than it was ten
years ago, arguably as a result of global warming.

And global warming may soon open new territories to invasion by the notorious
ragweeds. These tatty little plants with their green, tentacle-like flowers
belong to the genus Ambrosia (Latin for 鈥渇ood of the gods鈥� of all
things), and across much of the US and Canada they are responsible for more hay
fever than anything else. In recent years common ragweed, Ambrosia
artemisiifolia, has spread from its native North America to establish a
foothold in Japan and Europe, notably France. Its French stronghold is around
Lyon, where it was apparently introduced with American grain.

Ambrosiapollen turns up sporadically in pollen traps in London, but
at the moment Britain鈥檚 climate is too cool for the plant to set seed and
establish enduring populations. This could change. On current estimates, by 2050
the climate of Southeast England will be continental in character鈥攔ather
like Lyon is now, in fact.

鈥淭hese climatic conditions are likely to allow common ragweed to flourish as
well in Britain as it does in Europe,鈥� says botanist Tim Rich of the National
Museum of Wales in Cardiff. What鈥檚 more, says Rich, 鈥渟et-aside鈥�
land鈥攆ields allowed to go fallow to reduce surpluses of arable
crops鈥攚ill provide an ideal habitat for the plant, as has already happened
in neglected fields around Lyon and in the US. 鈥淎s global warming develops, the
spread of common ragweed and its associated allergies should be watched for by
botanists, aerobiologists and allergists alike,鈥� he warns.

Country drive

Aerobiology is also pinpointing newly discovered environmental hazards. In
the 1980s, spores from the common fern bracken were discovered to contain toxins
that can cause stomach cancer in animals that eat them. Might breathing them be
bad for humans? The spores regularly find their way into pollen traps in the
centre of Edinburgh, which suggests that few places in Britain are free of these
spores, says Eric Caulton of the Scottish Centre for Pollen Studies at the
city鈥檚 Napier University, who monitors the traps. He estimates that for two
months a year some people are inhaling up to 1800 spores in every cubic metre of
air they breathe.

It鈥檚 not just those who live, work or walk close to stands of bracken who may
be at risk. Compared with often-infertile stands of bracken in forests and
fields, plants on roadside verges tend to be packed with spores which could be
sucked into a car鈥檚 ventilation system. 鈥淭he most hazardous activity might turn
out to be not forestry or sheep-farming but a drive in the country on a sunny
day in late summer,鈥� he says.

To find out, aerobiologists like Caulton want to discover much more about
when, where and in what numbers bracken spores are present. 鈥淎erobiology can
enable us to interpret epidemiological data,鈥� says Stern. Where there is a
cluster of disease, for instance, it could help researchers discover whether
bracken spores might be to blame.

The vexed question of whether genetically modified crops can be kept separate
from non-modified crops is another area where aerobiology has a part to play.
鈥淭o think that the pollen of genetically modified plants can be contained in a
particular field is a gross misunderstanding,鈥� says Caulton.

Evidence for this comes from some of the earliest experiments in aerobiology.
They were conducted in the 1950s to track the fungal spores spreading potato
blight through a postwar Britain that was already suffering food shortages. Jim
Hirst, an agricultural researcher at what was then the Rothamsted Experimental
Station in Hertfordshire, took a pollen trap up in an aircraft and flew out over
the North Sea. Three hundred miles out to sea, he picked up bursts of pollen
from England. Pollen can clearly travel vast distances, although the biological
significance of that wanderlust is an open question.

Still, the profligate sexuality of plants clearly has a lot to answer for.
Luckily, whatever the dangers, aerobiologists are keeping a close watch.