Image provided by Alistair MacRobert via Unsplash

TL;DR: Indoor and ambient (outdoor) air pollution often come from different sources, but both are harmful and closely linked, with air moving between the two environments. Indoor sources include cooking fuels and certain building materials, while outdoor sources range from traffic to wildfires. Together, they contribute to millions of premature deaths each year, with ambient air pollution additionally harming ecosystems and urban landscapes. Regularly checking air quality and taking precautions during high-pollution periods can help keep you safe.

Ambient air pollution refers to air pollution found outdoors, while indoor air pollution occurs inside buildings and structures, often in households. Both are harmful to human health, and they share many similarities. However, they often come from different sources and carry separate concerns. 

Indoor air pollutants and sources

There are many potential sources of indoor air pollution. Some possible sources include the combustion of oil, gas, and kerosene. The burning of solid fuels such as coal, wood, charcoal, crop waste, and dung also harms air quality, along with open fires and poor ventilation. Smoking and tobacco products, certain building materials such as asbestos, and some cleaning products can all contribute to unhealthy indoor air. 

Poorly ventilated indoor fires contribute to indoor air pollution. This image is provided by andreas koulatsoglou via Unsplash. 

Common indoor air pollutants include particulate matter (PM), carbon monoxide (CO), and volatile and semi-volatile organic compounds. Burning coal can even emit sulfur oxides and heavy metals. 

Indoor air pollution concerns 

Since people spend much of their time indoors, it is important to ensure safe indoor air quality. Unfortunately, about a third of the world's population (2.1 billion people) cook using open fires or inefficient stoves burning kerosene, biomass, and coal. In 2020 alone, household air pollution was responsible for roughly 3.2 million deaths, disproportionately affecting lower-income countries. 

Fortunately, some things can be done to improve indoor air quality. Using cleaner fuels such as electricity and natural gas, utilizing safer technologies, and more efficient stoves can all help reduce household air pollution. Increasing ventilation and eliminating individual pollution sources (like sealing off asbestos) can also improve indoor air quality. High-quality air filters can additionally help purify the air. 

Luckily, the number of deaths attributed to household air pollution has been declining. There has been a reduction in the use of solid fuels for cooking. In 1980, 60% of homes participated in this practice, whereas only 42% of households did so in 2012.

This graph illustrates the annual number of premature deaths over the years attributed to household air pollution from the burning of solid fuels. The number of deaths has been declining globally. 

How Ambient air and indoor air affect each other

Much of the indoor air pollution we breathe actually comes from outdoors. Particulate matter from outdoor emission sources can seep through windows, doors, and HVAC systems, harming indoor air quality. Likewise, indoor air pollution sources can significantly contribute to ambient air pollution, affecting larger communities. 

Both sources impact human health. For instance, fine particulate matter (PM2.5), a major air pollutant both indoors and outdoors, is able to penetrate the lungs, damage lung tissue, and even enter the bloodstream. Exposure can aggravate lung disease, asthma, and result in acute bronchitis or susceptibility to respiratory infection. Particulate matter exposure can even lead to reduced lung function, chronic bronchitis, and premature death. 

Clarity Movement’s flagship Node-S air quality sensor measures ambient particulate matter (PM2.5) and nitrogen dioxide (NO2). 

Ambient air pollutants and sources

Prominent outdoor air pollution sources include both anthropogenic and natural sources. Some man-made sources include coal-fired power plants, agricultural production, and hydraulic fracturing. Fossil fuel combustion significantly contributes to ambient air pollution. Mobile sources, such as cars, account for more than half of the air pollution in the United States. 

Vehicle traffic significantly contributes to ambient air pollution. This image is provided by CHUTTERSNAP via Unsplash. 

Natural sources usually produce less air pollution than anthropogenic sources. However, sandstorms, wildfires, volcanoes, lightning, and the decomposition of organic matter can all deteriorate ambient air quality. 

The United States government has identified six harmful criteria air pollutants for which they have set National Ambient Air Quality Standards. These pollutants include carbon monoxide (CO), lead (Pb), Nitrogen Dioxide (NO2), Ozone (O3), both coarse and fine Particulate Matter (PM2.5 and PM10), and Sulfur Dioxide (SO2). Some more ambient pollutants include nitric oxide (NO), Black Carbon (BC), while ambient carbon dioxide (CO2) from fossil fuel burning is harmful to the environment. 

This table illustrates the current National Ambient Air Quality Standards as of July 2025. The passage of the Clean Air Act authorized the US EPA to put these standards into place.

Ambient air pollution concerns 

While ambient air pollution tends to disperse more than household pollution, which is more confined in an enclosed space, outdoor air pollution can still be incredibly dangerous, affecting communities, cities, and even spreading between countries.

In 2019, ambient air pollution is estimated to have caused 4.2 million premature deaths worldwide. Roughly 89% of those deaths occurred in low- and middle-income countries, with the WHO South-East Asia and Western Pacific Regions bearing the greatest burden. Yet, this type of air pollution is pervasive. In the same year, 99% of the world’s population was living in places where the World Health Organization (WHO) air quality guideline levels were not met. 

Outdoor air pollution often settles over densely populated areas in the form of smog. In addition to impairing visibility, smog can cause adverse health effects such as bronchitis and lung disease. It can exacerbate asthma, allergies, cause nausea, and even lead to premature deaths. 

Clarity’s Multi-Gas Module attaches seamlessly to our Node-S air quality sensor. This module detects a variety of ambient air pollutants, including carbon monoxide (CO), Ozone (O3), and nitrogen oxides (NOx), including nitric oxide (NO), and Nitrogen Dioxide (NO2).

Ambient air pollution also affects wildlife and ecosystems by contributing to acid rain, water eutrophication, and ocean acidification. Various ambient air pollutants can also reduce photosynthesis and deplete the necessary stratospheric ozone layer. While certain aerosol pollutants can mask global warming, others, such as greenhouse gases and black carbon, amplify it. 

Wildfires come with their own unique ambient air pollution burden. Wildfire smoke contains especially harmful particulate matter, especially when fires occur in Wildland-Urban Interface (WUI) areas, where burning buildings release harmful substances into the air, such as trace metals and asbestos. Moreover, in many parts of the world, wildfires are growing more intense due to climate change. Clarity is offering a discount on our monitoring equipment for the 2025 wildfire season.

Protecting against ambient air pollution

Be sure to check your local Air Quality Index (AQI) to determine when ambient air is safe. Both Clarity’s OpenMap and AirNow’s Fire and Smoke Map display local AQI. 

This graphic shows the different categories of AQI and how to respond to each of them. 

When the AQI is at an unsafe level, take proper precautions to ensure you stay healthy. Avoid pollutant sources and stay indoors. Close windows and doors to stop harmful outdoor air from coming in. Avoid any activities that might deteriorate indoor air quality as well. If you must go outside, wear an N95 or KN95 mask. 

Looking Forward

Indoor and outdoor air pollution are deeply connected, each influencing the other and shaping the air we breathe. Both significantly impair human health. Luckily, there are things that can be done to address these issues and make meaningful progress. Partner with Clarity to track ambient air pollution in real time and take action to ensure cleaner air for all.