breathing safely in CITY buildings

Created Wednesday, 18 October 2023


How do you best protect yourself from air pollution in a city environment? Is it safe to open a window to let the breeze in to cool your premises on a warm day? What about pollutants already present in your indoor air? 

According to the World Health Organization (WHO), 99% of the global population lives in areas
where the air is not safe to breathe. And city dwellers - who make up 56% or 4.4 billion of the
people in the world - suffer the most. An estimated 78% of carbon emissions and airborne
pollutants are generated in urban settings.  

The outdoor air in our cities contains air pollution from car exhausts, asphalt particles, industrial
processes, fires, dust, pollen, mould spores, etc. You also have sulphur dioxide which mainly
comes from burning fossil fuels in power plants. And ground-level ozone, which is formed when
nitrogen oxide, carbon monoxide and volatile organic compounds(VOCs) react with sunlight
– primarily in city environments. 

Indoor air is more polluted

It’s a warm summer day and the sun is shining and since you don’t have air conditioning, the
indoor air has become too hot. You open a window to let some air in to cool the premises.  

However, when outdoor air particles mix with particles already present in the indoor air, new
and often unknown compounds form that are potentially much more dangerous. In general,
indoor air is 2-5 times more polluted than outdoor air and can, in some cases, be up to
100 times more polluted.  Air pollution generated inside our buildings comes, for example, from:




air fresheners



aerosol propellants



paint & building materials

waxes & polishes


PM1 is the most harmful

Particulate matter that is 2.5 microns in size (PM2.5) is most talked about when referring to
air quality. But it is the ultrafine particles known as PM1 that cause the most damage to our
health. PM1 particles in our cities originate from car exhausts and other man-made sources.   

For optimal protection of people, a large focus should be put on these smallest air particles
that are 1 μm (micrometres) or smaller in diameter. PM1 is detrimental to our health because
the human body has no protection against these very tiny particles. We inhale them and a
significant part of them goes deep into our lungs and continues out into the blood stream.

At worst, PM1 contributes to heart attacks, lung cancer, dementia, emphysema, edema, and
other serious illnesses.  

How to stay protected

Luckily, there are effective measures you can take to protect yourself and the people around
you from being exposed to too much harmful air pollution in your building:   

1. Make sure your ventilation system is equipped with high grade HVAC air filters. We recommend 
Camfil’s City range of combined particulate and molecular filters that will effectively prevent more
than half of all PM1 particles (60%) from reaching your indoor air.   

2. Invest in one or several City range air purifiers to deal with the air pollution generated inside
your premises. They will additionally take care of pollutants that come in when you open the
window to let the breeze in to cool the indoor air.   

The City range of air purifiers also has molecular filtration that removes gases and odours from
your indoor space.   

Reducing the spread of viruses

A recent Irish study concluded that Camfil’s air purifiers have the capability to reduce the spread
of viruses through the air. According to the study, a City M air purifier removes aerosols from poorly
ventilated rooms at four times the normal rate. An ability that has been especially valuable during
the COVID-19 pandemic.   

Another study, conducted in the French region of Auvergne-Rhône-Alpes, showed that the City M
air purifier removed 99.9% of the COVID-19 virus in a given space. And that the virus did not
escape once it was trapped inside the unit and that it died after 48 hours.

Before opening a window

Our standing recommendation is to let the air filters in your building’s ventilation system (HVAC system)
clean the incoming air as much as possible. However, on the occasions when you do open a window to
let the breeze in, there are a few things you could consider:  

- Air pollution levels in our cities vary throughout the day and between different days and seasons.   

- A tip is to only open the window and let unfiltered air in when levels of particulate matter, nitrogen
oxide, carbon monoxide, volatile organic compounds and ozone are at their lowest.   

There are a number of online resources that allow you to check the air quality in your city or region
to determine when the outdoor air is least harmful.  

Here are a few examples:  

World’s Air Pollution: Real-time Air Quality Index 

European Air Quality Index  

Air Pollution in Asia: Real-time Air Quality Index Visual Map 


Explanations & sources

Ambient (outdoor) air pollution, by WHO
Urban development by the World Bank
- Irish study: Removal of aerosols in poorly ventilated spaces by University College Dublin and the Mater Hospital in Dublin
- French study in the region of Auvergne-Rhône-Alpes of City M removing the COVID-19 virus
- Volatile Organic Compounds (VOCs) are organic chemicals that are emitted as gases from certain solids or liquids. Examples of
indoor products that emit VOCs are paints, glues, cleaning supplies and furnishings. VOC concentrations can be up to ten times
higher indoors than outdoors, and they can have adverse health effects both in the long and short term.

For monitoring your indoor air quality (IAQ) we also recommend learning about Camfil’s Air Image Sensor System