Indoor vs. Outdoor Air Quality: What’s Worse + Why?

Human bodies don’t care whether they’re exposed to air pollution indoors or while we’re taking a walk outside. It all has a health impact. For this reason, more scientists and businesses are now focusing on the relationship between outdoor and indoor air pollution.

Let’s break down what these terms mean and why it’s so important to look at reducing exposure to poor air quality indoors and outdoors as a single challenge and not as two separate problems.

What Is Indoor Air Pollution?

When people think about ‘air pollution’ they tend to think about common outdoor sources, like fires, factories, and vehicle traffic. But indoor environments also have to contend with factors that can worsen air quality and pose a major threat to health.

Apart from obvious sources like cooking gasses and smoke or dust, the ‘culprits’ of indoor air pollution can also come from seemingly harmless household items like cleaning and personal care products, laundry detergent, paint, and pesticides.

What Are The Common Types Of Indoor Air Pollution? 

Because outdoor pollution can contribute to the presence of indoor contaminants, we can commonly find pollutants like VOCs, CO, NO2, SO2, and PM2.5 and PM10 indoors as well as outdoors. So if we ask, is indoor or outdoor air pollution worse, the answer will vary from person to person, depending on their local environment.

However, there are some pollutants that are typically only found indoors, for example, Radon, a radioactive product of radium, mold & fungi spores, and CO2, which is often used to measure the health of indoor environments.

Learn more about how to preserve and improve indoor air quality here

Causes Of Indoor Air Pollution That You May Have Never Thought Of

Most people expect air pollution to be something they are more exposed to outdoors than indoors.

However, according to the World Health Organization, indoor smoke from household air pollution presents a serious health risk for as many as 2.6 billion people (30% the entire world population!) who cook and heat their homes with biomass, kerosene fuels, and coal.

So we know that certain activities we do can cause harmful indoor air quality, such as candle burning, cooking, and using certain types of chemical cleaning products like bleach, the vapors from which can interact with VOCs or be broken down by direct sunlight to create other pollutants.

But as we mentioned earlier, it’s also possible for outdoor air pollution to migrate indoors through open doors, windows, ventilation shafts, and cracks.

Outdoor Air Pollution Migrating Indoors – Is Basic Protection Enough?

Unfortunately, it seems not. Researchers have found that some pollutants like PM2.5 can even seep through closed doors and shuttered windows, and concentrations can nearly triple during wildfire events. Luckily, newer buildings and ones utilizing air filtration systems seem to fare much better.

The exchange of air pollution also works the other way – according to the WHO, household air pollution can serve as a major source of outdoor air pollution in both urban and rural areas, accounting for as much as 50% of it in some parts of the world.

What is the Indoor-Outdoor Air Pollution Continuum?

Scientists today speak of the indoor-outdoor air pollution continuum to underline the fact that we shouldn’t focus on air quality indoors vs outdoors, or on each one of these aspects in isolation. Rather, we should think of indoor AND outdoor air pollution as two parts of a greater whole – our daily air quality exposure.

To understand the true picture of a person’s air pollution exposure, we need to consider the full 24-hour cycle of what an individual has actually breathed in.

Why is it Becoming More Important to Understand Individual Air Pollution Exposure?

We’re Creating More Pollution

Human beings have always been exposed to certain types of ambient air pollution like smoke, dust, and fires, but the addition of man-made sources of traffic pollution like industrial emissions has dramatically increased the overall health burden of air pollution exposure.

Today, the WHO estimates 99% of the world’s population regularly breathes in air that exceeds the WHO’s safe air quality guideline limits and that 4.2 million annual deaths occurred due to air pollution exposure in 2016 alone.

Seasonal Allergy is Getting Worse

When it comes to respiratory health, pollen exposure is also playing a larger role. Allergic rhinitis (hay fever) already affects between 10-30% of people around the world. Now, climate change seems to contribute to the increased duration and potency of pollen allergy season. Like traditional air pollution, pollen can also enter our homes when we open windows or hang our laundry outside, further impacting the health of indoor environments, especially for allergy sufferers.

Different pollen types

Common allergenic pollen-producing plants in North America

Unlike other forms of pollution, pollen exposure largely comes from the outdoors, again underlining the need to understand the connection between our indoor and outdoor environments.

We’re Spending More Time Inside

The trappings of modern life mean that people spend on average more than 90% of their time in an enclosed space (see study writeup here). Because we’re indoors so much, the indoor environmental air quality matters even more. Prolonged exposure to indoor air pollution even at lower levels can result in significant health impacts over the long term.

Indoor air pollution graph
Average indoor stay over a period of 24-hours (image source

What Can We Expect Going Forward?

Growing concerns regarding the health impacts of outdoor and indoor air pollution will likely make air quality monitoring as common as checking the weather. The COVID-19 pandemic, which left many of us working from home more often, certainly increased the focus on the impact of poor air quality exposure on health, leading to new baselines in air pollution mitigation strategies.

To keep up with rising demands for personalization and health, indoor air solution providers will rush to enhance their offerings by providing consumers with a 360 view into their personal environmental exposure along with tailored health-focused insights, applicable in real-time – at home, at work, and on the go.

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Eli Ginsburg
Eli Ginsburg

MSc in environmental technology and water management from Imperial College London. Former environmental consultant, specialising in modelling of air pollution and environmental impact assessments.