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Understanding How the Air Quality Index Works

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Infographic explaining how the air quality index (AQI) works

Understanding How the Air Quality Index Works

Air quality levels have received a lot of attention in recent years.

In the wake of COVID-19 lockdowns, many places reported a marked increase in air quality. Northern India captured the world’s attention when it was reported that the Himalayan mountain range was visible for the first time in decades.

On the flipside, later that summer, wildfires swept over the Pacific Northwest and California, blanketing entire regions with a thick shroud of smoke that spanned hundreds of miles.

How is air quality measured, and what goes into the health scores we see?

Measuring the Air Quality Index

When we see that air quality is “good” or “unhealthy”, those public health categories are derived from the Air Quality Index (AQI).

In the U.S., the AQI is calculated using five major air pollutants regulated by the Clean Air Act:

  • Ground-level ozone
  • Carbon monoxide
  • Sulfur dioxide
  • Particle pollution, also known as particulate matter
  • Nitrogen dioxide

Some countries have a slightly different way of calculating their scores. For example, India also measures levels of ammonia and lead in the air.

To make these readings more accessible, the AQI has a scoring system that runs from 0 to 500, using data collected from air monitoring stations in cities around the world. Scores below 50 are considered good, with very little impact to human health. The higher the score gets, the worse the air quality is.

To make communicating potential health risks to the public even easier, ranges of scores have been organized into descriptive categories.

AQI Score RangeAQI CategoryPM2.5 (μg/m³)Health Risks
0-50Good0-12.0Air quality is satisfactory and poses little or no risk.
51-100Moderate12.1-35.4Sensitive individuals should avoid outdoor activity.
101-150Unhealthy35.5-55.4General public and sensitive individuals in particular are
at risk to experience irritation and respiratory problems.
151-200Unhealthy55.5-150.4Increased likelihood of adverse effects and aggravation
to the heart and lungs among general public.
201-300Very Unhealthy150.5-250.4General public will be noticeably affected.
Sensitive groups should restrict outdoor activities.
301+Hazardous250.5+General public is at high risk to experience strong
irritations and adverse health effects. Everyone
should avoid outdoor activities.

Particulate Matter

While all the forms of atmospheric pollution are a cause for concern, it’s the smaller 2.5μm particles that get the most attention. For one, we can see visible evidence in the form of haze and smoke when PM2.5 levels increase. As well, these fine particles have a much easier time entering our bodies via breathing.

There are a number of factors that can increase the concentration of a region’s particulate matter. Some common examples include:

  • Coal-fired power stations
  • Cooking stoves (Many people around the world burn organic material for cooking and heating)
  • Smoke from wildfires and slash-and-burn land clearing

Wildfires and Air Quality

Air quality scores can fluctuate a lot from season to season. For example, regions that are reliant on coal for power generation tend to see AQI score spikes during peak periods.

One of the biggest fluctuations occurs during wildfire season, when places that typically have scores in the “good” category can see scores reach unsafe levels. In 2020, Eastern Australia and the West Coast of the U.S. both saw massive drops in air quality during their respective wildfire seasons.

Air quality in wildfire season

In June 2023, a storm system sent a thick blanket of smoke from Canadian wildfires down to Northeastern states, blocking out the sun and turning the sky over Manhattan into a dull shade of orange.

wildfire smoke raises AQI air qualities scores in NYC and other U.S. cities

Luckily, while these types of fluctuations are extreme, they are also temporary.

Correction: Graphics and article updated to include nitrogen dioxide.

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Visualizing The World’s Failure to Halt Deforestation

Global deforestation in 2022 rose by 4%, reaching 6.6 million hectares.

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Map Showing How the World Failed to Reduce Deforestation in Certain Regions Around the Globe, Primarily in Tropical Regions.

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The following content is sponsored by Carbon Streaming

Visualizing The World’s Failure to Halt Deforestation

Global deforestation in 2022 rose by 4%, reaching 6.6 million hectares. This number is 21% higher than the 2022 target needed to end deforestation by 2030.

In this map, our sponsor Carbon Streaming examines the failure to reduce deforestation in certain regions around the globe, based on data from the Forest Declaration Assessment.  

Most Deforestation Occurs in Tropical Regions

In 2022, deforestation alone accounted for around 7% of global greenhouse gas emissions.  

Nearly 96% of global deforestation took place in tropical regions in 2022. The loss of tropical primary forests reached 4.1 million hectares, 33% higher than the needed trajectory to halt primary forest loss by the end of the decade:

RegionDeforestation 2022, in thousand hectares (kha)Target for 2022* (kha)Deviation from target
Tropical Africa820.0650.0+26%
Tropical Asia1,930.01,900.0 +1%
Tropical Latin America and the Caribbean3,530.02,620.0+35%
Europe1.31.0+26%
Non-tropical Africa0.9 1.2 -22%
Non-tropical Asia18.320.9 -13%
Non-tropical Latin America and the Caribbean118.972.3 +64%
North America126.8 134.6-6%
Global6,560.0 5,510.0 +21%

Note: Based on original analysis for the Forest Declaration Assessment report using data from Hansen et al. 2013, updated through 2022. Only tree cover loss that is deemed permanent (Curtis et al., 2018) or that occurs within humid tropical primary forests is considered here. * Annual targets based on linear trajectory from a 2018-20 average baseline to 2030 target of zero deforestation.

Non-tropical forests in Africa and Asia, as well as forests in North America, suffered deforestation below the target for 2022.

Meanwhile, public and private finance for forests remains far below estimated needs for meeting global goals to halt and reverse deforestation.

More Funding for Forest   

Funding for forests averages $2.2 billion annually, representing less than 1% of the estimated requirements for achieving global forest goals by 2050.

Carbon credits can help mobilize the private sector capital needed to protect and restore forests by providing funds where it is urgently needed. Companies can purchase carbon credits to support critical mitigation efforts outside of their value chains, including nature-based solutions that may not receive funding otherwise.

Carbon Streaming has a portfolio of high-integrity carbon credit projects spanning 12 countries, including projects protecting forests such as the Cerrado Biome project and Rimba Raya project.

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You can make an impact by purchasing carbon credits from Carbon Streaming.

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