The Air Quality Index, or AQI, is a nationally uniform
index for reporting and forecasting daily air quality. It is used to report the
five most common ambient air pollutants that are regulated under the Clean Air
Act: ground-level ozone, particle pollution (or particulate matter), carbon
monoxide, sulfur dioxide, and nitrogen dioxide. The AQI tells the public how
clean or polluted the air is and how to avoid potential associated health
effects.
The
AQI uses a normalized scale from 0 to 500. Since levels rarely exceed a value
of 200 in the United States, in most cases only the range from 0 to 300 is
shown. The higher the AQI value, the greater the level of pollution and the
greater the health concern. An AQI value of 100 generally corresponds to the
level of the National Ambient Air Quality Standard for the pollutant. AQI
values below 100 are generally considered to be satisfactory. When AQI values
are above 100, air quality is considered to be unhealthy, at first for members
of susceptible populations, then for everyone as AQI values get higher.
The
AQI is divided into six categories that correspond to different levels of
health concern. For ozone, the breakpoints between these categories were selected
based on a review of the health effects evidence. This evidence included
concentration-response functions derived from a series of controlled human
exposure studies. (Folinsbee et al., 1988; Horstmann et al., 1990; McDonnell et
al., 1991). In these studies, healthy volunteers were exposed to ozone
concentrations ranging from 80 to 120 ppb for 8 hours in environmental
chambers. Activity patterns in these experiments were designed to simulate
activities of a normal day, with periods of moderate activity interspersed with
breaks for meals and for rest. Lung function and respiratory symptoms were
recorded for each exposure and graded based on Table 1 (for healthy people) and
Table 2 (for people with lung disease).
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Air
Quality Index
(AQI) values
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Levels of Health
Concern
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Colors
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When the AQI
is in this range:
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...air quality
conditions are:
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...as symbolized
by this color:
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0 to 50
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Good
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Green
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51 to 100
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Moderate
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Yellow
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101 to 150
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Unhealthy for Sensitive
Groups
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Orange
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151 to 200
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Unhealthy
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Red
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201 to 300
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Very Unhealthy
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Purple
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301 to 500
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Hazardous
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Maroon
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Figure 14: Air Quality Index (AQI) levels of health concern. This
color-coded table shows generically how AQI values correlate with levels of
health concern. For a copy of this AQI table, download the booklet, "Air
Quality Index - A Guide to Air Quality and Your Health," that explains
the AQI and the health effects of major air pollutants, at www.epa.gov/airnow/aqi_brochure_08-09.pdf. (629KB,
12 pp.)
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this figure
·
"Good" (0 - 50) Air
quality is excellent and poses little or no risk.
·
"Moderate" (51 - 100)Air
quality is acceptable; however, there may be some health concern for a small
number of unusually sensitive individuals. We cannot identify groups of
people that are at greater risk when air quality is in this range. However
controlled human exposure studies indicate that there are individuals who
experience health effects at more moderate levels of outdoor exertion or at
lower ozone levels than the average person, and these individuals may
experience effects when air quality is in the moderate range.
·
"Unhealthy for Sensitive Groups"
(101 - 150)When air quality is in this range, people that are included in a
sensitive group, whether the sensitivity is due to medical conditions,
exposure conditions, or inherent susceptibility, may experience the effects
described above when engaged in outdoor activities. However, exposures to
ambient concentrations in this range are not likely to result in effects in
the general population. For ozone, the sensitive group includes children;
people with lung diseases, such as asthma, chronic bronchitis, and emphysema;
older adults; and active people who work or exercise outdoors. EPA's risk
assessment (Whitfield et al., 1996) indicates that at this level for healthy
individuals (adults and children) at moderate exertion: 1) approximately 18%
are estimated to experience moderate or greater lung function impairment, 2)
approximately 9% are estimated to experience large or greater lung function
impairments, and 3) approximately 2% are estimated to experience moderate to
severe aggravated cough.
·
"Unhealthy" (151 - 200)When
air quality is in this range, any individual who is active outdoors may
experience the respiratory effects described above. Members of sensitive
groups are likely to experience more severe effects. EPA's risk assessment
(Whitfield et al., 1996) indicates that at this level for healthy individuals
(adults and children) at moderate exertion: 1) approximately 30% are
estimated to experience moderate or greater lung function impairment, 2)
approximately 15% are estimated to experience large or greater lung function
impairments, and 3) approximately 5% are estimated to experience moderate to
severe respiratory symptoms (i.e., chest pain with deep inspiration and
aggravated cough).
·
"Very Unhealthy" (201 - 300)When
air quality is in this range, it is expected that there will be widespread
effects among the general population and more serious effects in members of
sensitive groups. EPA's risk assessment (Whitfield et al., 1996) indicates
that at this level for healthy individuals (adults and children) at moderate
exertion: 1) approximately 50% are estimated to experience moderate or
greater lung function impairment, 2) approximately 20% are estimated to
experience large or greater lung function impairments, and 3) approximately
10-15% are estimated to experience moderate to severe respiratory symptoms
(i.e., chest pain with deep inspiration and aggravated cough). Individuals
with asthma or other respiratory conditions will likely be more severely
impacted than healthy individuals, leading some to increase medication usage
and seek medical attention, including increased emergency room and clinic
visits, and increased hospital admissions.
·
"Hazardous" (301 - 500) If air
quality gets in this range, it will trigger health warnings of emergency
conditions and there will be widespread coverage in the media.
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The AQI's levels of health concern correlate with
pollutant-specific health and cautionary statements that suggest relatively
simple measures people can take to reduce their exposure to air pollution. For
example, when the AQI Index Value for ozone is between 101 and 150, or Code
Orange, air quality is considered "unhealthy for sensitive groups,"
and active children and adults, and people with lung disease such as asthma, are
advised to reduce prolonged or heavy exertion outdoors.
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Index
Values
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Levels
of Health
Concern
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Cautionary Statements
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0-50
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Good
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None
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51-100*
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Moderate
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Unusually sensitive people should consider reducing prolonged
or heavy exertion outdoors.
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101-150
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Unhealthy for Sensitive
Groups
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Active children and adults, and people with lung disease, such
as asthma, should reduce prolonged or heavy exertion outdoors.
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151-200
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Unhealthy
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Active children and adults, and people with lung disease, such
as asthma, should avoid prolonged or heavy exertion outdoors. Everyone
else, especially children, should reduce prolonged or heavy exertion
outdoors.
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201-300
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Very Unhealthy
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Active children and adults, and people with lung disease, such
as asthma, should avoid all outdoor exertion. Everyone else, especially
children, should avoid prolonged or heavy exertion outdoors.
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301-500
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Hazardous
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Everyone should avoid all physical activity outdoors.
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Figure 15: The AQI Guide for Ozone
includes cautionary statements and actions people can take to reduce their
risk from exposure to air pollution at different levels of health concern.
This brochure, can
be found online and it includes AQI tables for other air pollutants.
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this figure
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What
can I advise my patients to do when air quality is forecast to be unhealthy?
As
discussed above, the EPA recommends limiting outdoor activities as the AQI for
ozone rises to unhealthy levels. The chances of being affected by ozone
increase the longer a person is active outdoors and the more strenuous the
activity is. If a patient is planning to engage in an activity that requires
moderate or heavy exertion, they can reduce the time spent on this activity or
substitute another activity that requires less exertion (e.g., go for a walk
rather than a jog). In addition, patients can plan outdoor activities when
ozone levels are lower, usually in the morning or evening. Check State or local
air quality agency's air quality websites for the most up to date information
on local pollution levels or visit www.airnow.gov.
In
general, controlled human exposure studies indicate that there is a risk of
experiencing respiratory symptoms and reduced lung function at the following
ozone concentrations and exertion levels:
·
At
ozone levels above 120 ppb and heavy outdoor exertion for short periods of time
(1 to 3 hours)
·
At
ozone levels between 60 and 80 ppb and intermittent moderate outdoor exertion
for longer periods of time (4 to 8 hours)
For
most adults, activities that involve moderate exertion, with ventilation rates
for adults that range from 35-45 liters/minute, include climbing stairs,
playing tennis or baseball, simple garden or construction work, and brisk
walking or light jogging, cycling, or hiking. Activities that involve heavy
exertion, with ventilation rates greater than 45 liters/minute for adults,
include playing basketball or soccer, chopping wood, heavy manual labor such as
digging in the garden, and vigorous running, cycling, or hiking. Because
fitness levels vary widely among individuals, what is moderate exertion for one
person may be heavy exertion for another. No matter how fit a person is,
cutting back on the level and/or duration of exertion when ozone levels are
high will help protect him or her from ozone's harmful effects.
The
appearance of respiratory symptoms when ozone levels are high is an indication
that reduction of exposure is warranted. These symptoms can include:
·
Coughing
·
Throat
irritation
·
Pain,
burning, or discomfort in the chest when taking a deep breath
·
Chest
tightness, wheezing, or shortness of breath
In
general, children are less likely to notice or report symptoms than adults,
even when measures of peak flow are being affected.
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