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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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Ranked: The Most Carbon-Intensive Sectors in the World

Comparing average Scope 1 emission intensities by sector according to an analysis done by S&P Global Inc.

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Ranked: The Most Carbon-Intensive Sectors in the World

This was originally posted on Elements. Sign up to the free mailing list to get beautiful visualizations on real assets and resource megatrends each week.

Ever wonder which sectors contribute the most to CO2 emissions around the world?

In this graphic, we explore the answers to that question by comparing average Scope 1 emission intensities by sector, according to an analysis done by S&P Global Inc.

Defining Scope 1 Emissions

Before diving into the data, it may be useful to understand what Scope 1 emissions entail.

Scope 1 emissions are direct greenhouse gas emissions from sources that are owned or controlled by a company, such as their facilities and vehicles.

Source: U.S. Environmental Protection Agency

Scope 1 emissions can do a good job of highlighting a company’s environmental footprint because they represent the direct emissions related to manufacturing or creating a company’s products, whether they are tangible goods, digital software, or services.

Scope 2 and 3 emissions, on the other hand, encompass the indirect emissions associated with a company’s activities, including those from a company’s purchased electricity, leased assets, or investments.

Ranking the Carbon Giants

According to S&P Global’s analysis of 2019-2020 average emissions intensity by sector, utilities is the most carbon-intensive sector in the world, emitting a staggering 2,634 tonnes of CO2 per $1 million of revenue.

Materials and energy sectors follow behind, with 918 tonnes and 571 tonnes of CO2 emitted, respectively.

SectorSector ExplanationScope 1 CO2 emissions per $1M of revenue, 2019-2020
UtilitiesElectric, gas, and water utilities and independent producers2,634 tonnes
MaterialsChemicals, construction materials, packaging, metals, and mining918 tonnes
EnergyOil and gas exploration/production and energy equipment571 tonnes
IndustrialsCapital goods, commercial services, and transportation194 tonnes
Consumer staplesFood, household goods, and personal products90 tonnes
Consumer discretionaryAutomobiles, consumer durables, apparel, and retailing33 tonnes
Real estateReal estate and real estate management31 tonnes
Information technologySoftware, technology hardware, and semiconductors24 tonnes
FinancialsBanks, insurance, and diversified financials19 tonnes
Communication servicesTelecommunication, media, and entertainment9 tonnes
Health careHealth care equipment, pharmaceuticals, biotechnology, and life sciences7 tonnes

S&P Global also reveals some interesting insights when it comes to various industries within the materials sector, including:

  • Cement manufacturing exhibits an extremely high level of Scope 1 emissions, emitting more than double the emissions from the utilities sector (5,415 tonnes of CO2 per $1M of revenue)
  • Aluminum and steel production are also quite emission-intensive, emitting 1,421 and 1,390 tonnes respectively in 2019-2020
  • Relatively lower-emission materials such as gold, glass, metals and paper products bring down the average emissions of the materials sector

Given these trends, a closer look at emission-intensive industries and sectors is necessary for our urgent need to decarbonize the global economy.

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