Understanding the Air Quality Index (AQI)
An air quality index indicates the average level of pollution in the air over a period of time. AQI systems vary by country and may represent differing types of pollutants. NIST uses the US AQI standard, reported in real-time readings.
0-49
Good
50-99
Moderate
100-149
Unhealthy for sensitive individuals
150-174
Unhealthy
175-299
Very unhealthy
300+
Hazardous
Our AQI Policy
Though research into long-term impact of air pollution in relation to particulate matter (PM) is ongoing, it is clear that exposure to PM of 2.5 microns and smaller can be very harmful. However, it is important to note that the impact is based upon the length of exposure. The EPA and other organizations indicate that higher AQI levels (100-200) are primarily harmful when outdoors for an extended length of several hours and when being moderately active.
Though all of us are impacted by air pollution, children are particularly vulnerable. In line with the recommendations of the World Health Organization and US Environmental Protection Agency, we maintain a clear policy to ensure the wellbeing of all students and staff. NIST maintains multiple air quality meters on campus that allow us to continually track levels of pollution without relying solely on public sources and, when necessary, take appropriate action to safeguard children.
Each of the thresholds for the modification or cancellation of outdoor activities, and shift to indoor learning, is provided below, as well as in greater detail in our policy document. Additionally, the key source for our policy, A Guide to Air Quality and Your Health, is accessible on this page.
What Impacts the AQI & Pollution Levels?
It is important to understand that there is no single standard of measuring air quality, as every country uses a different method based on calculations for types of pollutants. The most commonly used system, due to its clarity and more stringent standards, is that of the United States. This includes measures of carbon monoxide, nitrogen dioxide, ozone, particulate matter (PM10 and PM2.5), sulfur dioxide. The US AQI is used on many public websites and here at NIST, with the highest pollutant at any given time used to calculate the reading.
Accessing accurate AQI readings can also be a challenge. Some websites such as aqicn.org rely on all public government stations, normalized to USA AQI standards & weighted toward recent changes. In contrast, another like airvisual.com combines those same public stations with readings from satellite modeling and privately owned devices set to contribute data. In both cases, the sites aggregate readings from either the previous 24 hours, or based on the update cycle of contributing devices, to provide an average for the previous one hour. These different calculations can mean very different AQI levels depending on the source.
Air pollution is similarly strongly impacted by a large number of factors, and can vary even in areas in very close geographical proximity. Wind, atmospheric conditions, building height and density, traffic patterns, fires, factories, trees and foliage, and other emissions can all potentially change AQI readings by 2 to 15%, or more in some cases.
Urban planning & development
“..pedestrian-level traffic-related pollutant concentration depends on the permeability of the entire street canyon.” (Source)
Atmospheric conditions
“Temperature and wind speed were the most important determinants of PM2.5 concentrations and trends in most regions.” (Source)
Urban foliage & vegetation
“Effects of vegetation removing pollutants from urban air…have been demonstrated in field measurements and using models.” (Source)
Open crop burning
“…agricultural residue burning has negative impacts on the environment, specifically air pollution and climate change.” (Source)
Transportation industry
“The global transportation sector is a major source of this…through its contribution to elevated fine particulate matter (PM2.5)…” (Source)
Industrial emissions
“…industrial emissions may importantly contribute to ambient air pollution experienced by some local populations.” (Source)
Which device does NIST use to measure air quality and why?
To determine AQI levels on the NIST campus, we use AirVisual Pro monitors. This particular device offers numerous advantages, including highly accurate sensors and real-time monitoring. Equally important, it allows comparative readings with other devices that have been set to share data on the AirVisual website. Although only one NIST monitor is set to allow for public viewing, several other devices are placed throughout the campus and are used to compare the readings for accuracy. When any one device indicates an AQI level significantly different from others, it is sent to an authorized repair centre for recalibration.
When does the NIST policy go into effect?
Any AQI threshold that leads to modification of outdoor learning or a full shift to indoor learning will be triggered following three full update cycles of the AirVisual Pro device, equivalent to a period of 15 minutes. This guideline is in place due to the rapid changes that can occur in AQI levels over a short period of time due to factors such as idling vehicles or changes in airflow.
If the AQI level remains above a threshold level for 15 minutes, the corresponding policy goes into effect, and faculty are notified. If the AQI level reaches significantly higher levels, the parent community will also be informed. Finally, in some cases in which pollution levels exceed 200, NIST and other schools may be directed to close their campuses by the Ministry of Education, at which time we will shift to online learning.
Is it dangerous to be outside if the AQI is high?
Although all air pollution is unhealthy regardless of the concentration, short-term exposure at moderate levels typically does not have a lasting impact. For this reason A Guide to Air Quality and Your Health and other government sources do not specify that people should stay indoors at those levels, but rather that they should avoid prolonged exertion. This is defined as outdoor activity that lasts several hours and causes breathing heavier than normal.
It is important to note that even at AQI levels above 150 and below 200, the EPA simply recommends that sensitive groups, including children and older adults, avoid prolonged exertion for periods of several hours. However, we prefer to begin moving all students indoors at 175, and limit outdoor exposure in order to safeguard their wellbeing.
How does NIST ensure that indoor AQI levels remain low?
Several measures are in place to improve indoor air quality. The Sports Complex and Creative Arts Building have central air systems with HEPA filters. In other buildings we use LG PuriCare air purifiers, which filter particulate matter down to one micron (PM1.0), in order to reduce PM2.5 pollution. Based on tests conducted throughout days with high AQI levels, these purifiers consistently maintain PM2.5 levels below the World Health Organization’s recommended 24-hour average of 25. We track readings in learning areas on a daily basis during periods of high pollution to ensure that levels remain low.
Additional Resources
A collaborative effort of the United Nations, World Bank and World Health Organization, Breathelife provides information about individual actions we can take to improve global air quality.
Our World in Data’s page on air pollution provides a top-level view of its widespread impact around the world, including the immense health burden that it generates.
As a global non-profit represented by nearly 1,000 scientific and business experts, the World Resources Institute has taken a lead in preserving natural resources and combating pollution.
