The Invisible Threat: What’s Really in Your Indoor Air?

Health Impact

April 28, 2025

Would you believe someone if they said the air in an urban street with average traffic might actually be cleaner than the air in a living room?

Surprisingly, indoor air can be two to five times more polluted than outdoor air and sometimes even worse (World Health Organization).

In a world where we spend up to 90% of our time indoors (Carslaw, 2007), understanding the hidden risks in the air we breathe has never been more important.

Let’s uncover what’s floating in the air around you and why it matters.

This article is Part 1 of our three-part series on indoor air quality. In the next posts, we'll explore how poor air impacts growing children and what solutions can help us breathe easier.

What Is Indoor Air Pollution?

Indoor air pollution refers to the buildup of harmful substances inside enclosed spaces like offices, stores, homes, and schools.

These pollutants can come from outside (like traffic emissions) or be generated indoors through everyday activities such as cooking, cleaning, or off-gassing from furniture and building materials (González-Martín et al., 2020).

Indoor air is often worse than outdoor air because of:

Poor ventilation that traps pollutants inside.
Off-gassing from furniture, paints, and construction materials.
Chemicals from cleaning products lingering in the air.
Human activity, releasing CO₂, bacteria, and viruses into enclosed spaces.

Without enough fresh air flow, these pollutants can accumulate to levels far exceeding outdoor concentrations - even in new or well-maintained buildings.

Poor indoor air quality costs billions of euros every year across Europe through lost productivity and increased healthcare expenses (WHO Regional Office for Europe, 2015).

The Common Indoor Pollutants You Should Know About

Here are the main invisible culprits affecting indoor air:

Table: Pollutants, their main sources and health impact

A recent review found that over 400 different chemical compounds can be detected indoors - many known or suspected to harm human health (González-Martín et al., 2020).

Bacteria, Viruses, and the Invisible Spread Indoors

Indoor air isn’t just a container for chemicals and particles, it’s also a transport system for bioaerosols:

Bacteria and viruses can stay suspended in air for hours in unventilated spaces (NRC, 2004)
Respiratory diseases, like RSV, influenza, and COVID-19, spread far more efficiently in poorly ventilated classrooms and shared facilities (RCPCH, 2020)
Mould spores, in particular, are a proven asthma trigger and can cause infections in immunocompromised children (PCRU, 2024)

Proper ventilation and air filtration significantly reduce airborne pathogens, which is a crucial step in public health management.

How Bad Is It Indoors? Lessons from Daycares, Schools, and Offices

Studies across Europe, including Belgium, paint a concerning picture.

In daycares and schools globally, air quality often fails to meet health guidelines:
- CO₂ concentrations frequently exceed 1000 ppm, a key sign of poor ventilation (Anake et al., 2023).
- High levels of dust (PM₂.₅ and PM₁₀), VOCs like formaldehyde, and outdoor pollutants like NO₂ have been detected indoors.
- Poor air quality has been linked to more frequent respiratory problems, increased absenteeism, and lower cognitive performance in children (Sadrizadeh et al., 2022).
In Belgium specifically, extensive studies found:
- Indoor VOC levels exceeded recommended guidelines in the majority of tested homes, offices, and schools.
- In daycare centers in Ghent, indoor air contained 5 to 6 times more VOCs than outdoor air (De Coster et al., 2023).
- Hazardous chemicals like benzene (a known carcinogen) were detected both indoors and outdoors.

While air pollution in Flanders has improved in recent years, many areas still fail to meet WHO health recommendations.

To tackle indoor air challenges, Belgium introduced a national ventilation plan, requiring CO₂ meters in public spaces and recommending CO₂ levels below 900 ppm (Level A) and below 1200 ppm (Level B) (International Futures Forum, 2024).

In short: poor indoor air is widespread - and it affects everyone, from children and employees to customers.

Awareness is the First Step

The good news? Awareness is power.

Knowing what’s in your air is the first step toward creating healthier spaces for your employees, your customers, and your loved ones.

In our next post, we’ll dive deeper into how poor indoor air specifically impacts growing children and why early detection is crucial.