We have indoor air quality monitors and purifiers to manage indoor air, but even the most intelligent domestic devices cannot inspect all the air pollutants present in our indoor spaces, so I thought it was about time to send a filter from the main air purifier to a laboratory for analysis in order to identify all the pollutants that have been present in my house. Maybe there is something I am not aware!
I try my best to keep outdoor pollutants from entering inside like smoke from wood-burning stoves or fumes from vehicle traffic. However, we are humans and we make mistakes, so sometimes we burn the food or introduce pollutants indoors unconsciously.
We humans can smell some pollutants like PAHs or VOCs but sometimes we are unable to detect heavy metals or other pollutants that our nose isn’t able to register, yet we may breathe them. Air purifiers or central HVAC with a filtration system capture those pollutants in the filter media. Then we can analyze the filter and discover aspects of the pollution we hadn’t thought about before.
Camfil is a producer and developer of air filters and clean air products. Camfil is also a global air filtration leader with 24 production units and R&D centers in four countries in the Americas, Europe, and the Asia-Pacific region. My filter was shipped to the R&D labs in Sweden where a lab engineer (thank you Filip) did all sort of experiments.
Scanning electron microscope (SEM)
The lab uses a Scanning Electron Microscope (SEM) which uses focused electrons to generate an image and it is possible to enlarge samples up to 300,000x magnification, something not possible with optical microscopes. The equipment also features an EDS (Energy-Dispersive Spectroscopy) detector which can reveal the chemical information from various particles present in the sample. The lab conducts different analyses by using three methods.
1st Air analysis
During this analysis, a vacuum pump is used for sampling the particles onto a membrane, trapping the particles to a sample holder. Afterward, the sample is analyzed by SEM imaging, chemical/elemental analysis (EDS/EDX, X-ray detection), and overall appearance (particle load high or low) of the air sample(s) based on previous experience taken from thousands of different air samples in different environments over 20 years.
2nd Media analysis
When doing the analysis the media sample is attached to a suitable sample holder. The surface is then analyzed by the EDS-detector which utilizes the shape, size, detected elements, and topic structure. This makes it possible to generate an image showing the different elements present in the sample and where they are located in the sample.
3rd Dust/material analysis
When doing the analysis the material/dust is attached to a suitable sample holder. The surface is then analyzed by an EDS detector. This provides information on what elements are present in the sample. In some cases, it is possible to perform an automated particle detection that classifies particles based on shape, size, and detected elements.
In my case, the lab did a media analysis. Also, they were able to determine the type of the finest filter based on the size of the fibers and the space between them.
As we can see from the images there is a chaotic network of fibers with various particles attached to them. The lab analyzed a random portion of the filter and they found all sorts of stuff. As the SEM was zooming in, more and more details were revealed about the particles. Take as an example the image below where an agglomeration of nano-sized combustion particles hung out. The origin of these particles is most likely from a Diesel engine. Thankfully it didn’t end up inside me.
The automatic particle identification, which is part of the EDS, was able to analyze and classify the particles from a tiny portion of the filter. Here is a list of the count number and size of the particles with an explanation.
According to the expert, the measured fiber size of the finest filtration layer of the purifier media is too large to be able to achieve actual true HEPA classification. It does not pass HEPA test protocol which is EN1822:2019 standard. HEPA filters usually have fiber sizes ranging between <100nm-500 nm. On the other hand according to the manufacturer, the media is an H11 HEPA filter and uses synthetic fibers instead of fiberglass. The lab engineer classified it as an ePM2.5 70% and ePM1 50% F7 class media. The filter was able to capture particles with an aerodynamic diameter of 2.5 microns efficiently but not at 0.4 microns. Keep in mind, the filter is 2 years old and humidity has degraded it over time.
All things considered, I strongly believe that such a deep analysis of the “dust” accumulated on a filter offers a better understanding of the sources of indoor air pollution and how we can mitigate it. In my case, I didn’t know that building materials are accumulating in such a high concentration inside my house and I may reevaluate some of my choices regarding the drywalls. Saharan dust particles are also present, but I am not sure about the origin of the sulfur particles the EDS detected. A theory is that they may have come from the volcano in La Palmas or from the combustion of the unregulated fuels ships use here at the city port. For more information about the SEM analysis please visit Camfil here.