Environmental Epidemiology is the study of health consequences of exposures
that are involuntary and occur in the general environment such as air pollution,
noise, and pollutants in water.
Outdoor Air Pollution also called Ambient Air Pollution is the most
studied environmental pollutant.
Air pollution epidemiology is typically divided into two areas.
Studies of health effects related to acute or
short term exposures to air pollution over hours,days or weeks.
And studies of health effects of chronic or
long term exposures to air pollution over years, decades or a lifetime.
The earliest epidemiological studies are those that evaluated air
pollution disasters or episodes.
These studies compare mortality and morbidity before, during and
after air pollution episodes.
When unusually high levels of pollution clearly lead to unusual high
levels of mortality and/or morbidity.
One of the first documented air pollution disasters was
the Great Smog of London in 1952, also know as the Killer Fog.
This was a severe air pollution event affecting London for four days,
from 5th to 9th of December, when cold weather combined with windless conditions
caused airborne pollutants, mainly from use of coal to form a thick layer of smog.
The smog caused major disruption by reducing visibility,
penetrating indoor areas, and causing health effects.
The association between air pollution and
health during this episode was evident as a sharp, five fold increase in sulfur
dioxide levels was immediately followed by sharp increasing mortality.
It was estimated that 12,000 died directly as a result of the London Smog.
Similar episodes but at a smaller scale occurred in
December 1930 in Belgium and in 1948 in USA.
However, the London Smog of 1952 presents a landmark in air pollution epidemiology.
Because of the scale of disaster and because it provided data to researchers
to perform detailed analysis for the first time.
These episodes of the 1930s to 1950s made important impact on science,
public perception of air pollution, and government regulation.
Today, such high pollution events are no longer seen in Europe and
the United States, mainly due to reductions in use of coal, and
introduction of central heating in the cities.
Since the 1990s the focus in air pollution epidemiology has been on
health effects related to low air pollution levels, and
traffic-related air pollution has gained increasing importance.
Studies of air pollution episodes are still relevant in the areas of the world
with high and rising air pollution levels, such as China and India.
The study design used to study health effects of acute exposures to air
pollution is a so-called Time-Series Study.
In these studies, typically based in a single large city,
data on daily concentrations of an air pollutant from a central
monitor are linked to data on a daily count of deaths or hospitalizations.
As an example, we can look at this time series of daily background
concentrations of nitrogen dioxide from a background monitor
in Copenhagen between 2001 and 2009.
Notice, that the concentrations are higher in the winter periods.
And here are the daily number of asthma hospitalizations in children
in Copenhagen hospitals during the same period.
The study found that for each seven parts per billions increase in main
nitrogen dioxide levels over five days,
there was a 10% increase in the risk of asthma hospitalization the following day.
In time series studies, all people living in the given location or
city are assumed to have the same air pollution exposure.
And it is the daily variations in pollution levels over time that drive
the daily variations in a health outcome.
Long-term studies are needed to provide answers as how to much life is
shortened due to air pollution.
Whether air pollution affects long term mortality rates in a given population,
or is it a risk factor for a disease?
In these studies, subjects living in areas with different air pollution
levels are followed for a long period of time, years or even decades to determine
whether people living in areas with high pollution, develop more disease or
die earlier than the people living in low air pollution areas.
These are typically called Cohort Studies.
In such studies, it is very important to adjust for
other personal characteristics that are related to mortality and
morbidity such as smoking, physical activity, and education.
One of the earliest and most famous air pollution studies is
the Harvard Six City Study, a perspective cohort study of about 8,000 subjects
selected randomly from six US cities with different levels of air pollution.
Analysis compared mortality rates in each city from 1974 until 1991 and
found increasing mortality with increasing level of particulate matter.
The most polluted city Stubenville, Ohio had 26% higher
mortality than the least polluted city, Portage, Wisconsin.
Another reason example is a large nationwide study from the United States on
60 million Medicaid beneficiaries from 2000 to 2012.
Which linked all cause mortality data to annual averages of PM 2.5 and
ozone at a zip code level.
The study showed that each increase of 10 micrograms per cubic meter and
PM 2.5, and of 10 PPP in ozone associated with increase in
all cost mortality of 7.3% and 1.1% respectively.
Combining data from several studies is seen as an advantage for
better power and precision in estimating air pollution related health effects.
An example is a large European study of cohorts on air pollution effects,
so called Escape.
In this study PM 2.5 levels at home were linked to mortality data for
approximately 367,000 people from 22 European
cohorts from 13 countries during 14 years.
They found that for each increase of 5 micrograms per cubic meter in PM 2.5,
there was a 7% increase in all cause mortality.
Some of the challenges future epidemiological studies will address
are studying effect of complex exposures to multiple pollutants.
Studying effects of short & long term exposures simultaneously.
And incorporating novel air pollution exposure modelling and measurement method,
such as personal monitoring, satellite based models data and so on.
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