What the Scientists tell us

There is considerable research in academia on ambient air quality. However, ambient refers to outdoor air. Most of the research consists of statistical analysis of hospital admissions for respiratory problems or mortality rates on days when there is extreme air pollution.

The statistical analysis consists of what scientists call meta-analyses (systematic reviews) of single-city time series studies. Basically an average of multiple studies.

The only indoor air studies are those that investigate the effects of indoor open fire cooking.These are important studies, mostly by the World Health Organization (WHO), but they only apply to underdeveloped countries where these practices are unfortunately still prevalent. They are not relevant to indoor conditions in the modern world.

Scientists are still divided on what standards should be imposed on ambient air. The U.S. Environmental Protection Agency (EPA) promulgated the National Ambient Air Quality Standards (NAAQS) designed to establish regulation of fine particulate pollution. But other scientists believe that these standards are too lax, with allowable pollution levels above the standards of the World Health Organization (WHO).

The science of clean air is not settled.

The statistical studies of ambient air have, however, one important conclusion. They tell us that the pollution effect on human health depends in part on the size of the particles in the air.

In this case size matters.

We can use this information when we address indoor pollution solutions.

But improving indoor air quality in our homes is our individual responsibility.

We cannot wait for solutions from the academic world.

Indoor Pollutant Particles

Particulate matter (PM) air pollution is an air suspended mixture of solid and liquid particles of various sizes. Scientists divide the particle sizes in 3 categories.

Coarse Particles – larger than 2.5µm (PM10)

Fine Particles – smaller than 2.5µm (PM2.5)

Ultrafine Particles – smaller than 0.1µm included in (PM2.5)


  • The size is the diameter of a sphere that has the same aerodynamic properties as the particle. Since particles vary in shapes, using an equivalent sphere diameter is useful in describing pollutant characteristics.
  • A µm or “micron” is a measure of length equal to one millionth of a meter, or one thousandth of a millimeter.

It is helpful to know what pollutants are included in the above categories. This may help us in deciding what action we should take to mitigate the health risks.

Coarse particles include dust, pollen, mold, spores, fly ash, smoke

Fine particles include volatile organic compounds (VOC) such as benzene, toluene and formaldehyde from smoking, cooking, gas stoves, cleaning fluids, solvents, pesticides

Ultra-fine particles originate in vehicle exhaust and atmospheric photochemical reactions, viruses

In the Relationship of Indoor, Outdoor and Personal Air (RIOPA) study, based on measurements at 173 homes in Houston, Los Angeles County and Elizabeth, NJ the researchers found as one of their conclusions that the mean particulate organic matter concentration indoors (9.8 µg/cubic meter) was twice the mean outdoor concentration ( 4.9 µg/cubic meter ).Furthermore on average 71% to 76% of indoor Organic Matter (OM) present in the homes was emitted or formed indoors.

Health Effects of Indoor Pollutants

Current research indicates that fine and ultrafine particles are the most harmful to human health, since they can more easily penetrate the lungs and they also transport toxic chemicals. Furthermore, ultrafine particles are more likely to translocate from the lungs to the blood stream. It should be our first priority to eliminate this size of particles (PM2.5) from our indoor environment.

There are many studies showing the health risks associated with environmental tobacco smoke (ETS).Estimates indicate that 3000 lung cancer deaths each year can be attributed to passive smoking in the USA as well as hundreds of thousands of childhood respiratory disease cases. Globally (ETS) is perhaps the largest modern source of indoor pollution. However there are not many studies on the health effects of inhaled biological particles or the various (VOC) present indoors. We know much less about the health risks from indoor pollution than we do about those attributable to the contamination of outdoor air.

We can Take Action

Although we may not know the specific health risks in our home environment common sense dictates that we take action that is possible.

We can minimize the use of the pollution sources listed above such as (VOC) and select an air purifier.


In selecting the air purifier we must carefully review the manufacturer’s test data to confirm that (PM2.5) particles will be removed.

Improving indoor air quality in our homes is our individual responsibility.We can take action now.The air purifier that I recommend is also an air quality monitor.






I will be pleased to answer any questions.You can reach me at





  1. Well, I am surprised that indoor air quality is worse than outdoor air quality in term of small particles. I think this is a condemnation of modern sealed houses.

    It sounds like a good air purifier is called for in homes and offices that constantly keep their window seal for efficiency.

    I live in a town with relatively good, fresh air, except for organic allergins. We don’t need central heating, but we do use air conditioning in the warm weather. I might want to invest in an air purifier for the bedroom, but elsewhere, I will just open the windows to even out the particles between our home and the environment.

    1. You are fortunate that you live in an area with good outdoor air quality.Ventilation by opening windows for a few hours maybe is all you need.Still, you need to be aware of the pollutants you create indoors through cooking,smoking ,etc. Indoor air quality in our homes is our responsibility.We must be vigilant.A good air monitor would also help.

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