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G. Carboni, November 2004
Text editing by Donald Desaulniers, Ph.D.







When it comes to housekeeping chores, what is more annoying than dust? Dust is everywhere! It seems to enter from fissures and cracks and then settles out on almost everything in sight. A few days without dusting are enough for one to be able to observe fine dust deposited everywhere, especially on the dark furniture.. With time, the dust will become thicker and more noticeable. For people who are particularly sensitized or allergic to dust, certain kinds of dust can cause asthma and even bronchitis. However, for the amateur microscopist who is always looking for samples to observe, dust can be a fascinating area of study, especially when the focus is on the nature and origin of airborne particles.

Samples of dust can be collected in many different environments where they will exhibit different compositions. For example, on an open surface in a natural environment within continental Europe, you can expect to find fragments of plants, moulds, insects, pollen, spores, microorganisms and mineral dust. You may even find dust from the Sahara desert or micrometeoric dust from outer space. In a forested area, biological detritus is common, while in a desert environment, the mineral particles is more prevalent. In urban environments near cities, you will mainly find particles from the emissions of motorized vehicles and from the power plants. While the quantities are minute, you will also find particles released from the wearing of brake linings and rubber tires as well as mineral particles. In the floor sweepings you collect from a dinning room with a broom, you will likely find bread crumbs, sand grains including human and pet hairs. In the dust collected on a bookcase you should find textile fibers and thin fragments of biological and mineral material. In a bedroom, dust should be richer of textile fibers, but you may also find dust mites and their dreaded excrements. In machine shops, laboratories and factories the composition and the nature of the dust particles will vary according to the type of activities conducted there.


How do you collect dust samples to observe with a microscope? If the dust has settled on a smooth surface, you can collect it with a clean finger, or you can transfer it onto a microscope slide with a small brush. Perhaps, the best way is to use a pipette to apply water on the dusty surface and then collect your sample using the pipette . You can repeat this operation several times using the same water until you have collected a sufficient quantity of dust suspended in the water. Another method is to place a microscope slide in a selected location and wait until enough dust has settled on it. Obviously, you should not be in a hurry if you are going to use this method of sampling! If the quantity of available dust you want to collect is small and you do want to avoid contaminating it, you can use an adhesive tape to collect your sample.

Yet another method consists in using a vacuum cleaner with a filter placed over the intake of the hose. If the filter causes too much of a restriction, this can strain the motor of the vacuum cleaner and eventually burn it out. To avoid damaging your vacuum cleaner, you should use a porous filter media, or allow some air to bypass the filter. It is possible to find vacuum cleaners on the market that are equipped with a specially liquid chamber to filter the air. If you have one of these units, after cleaning your carpets, you can collect a sample of the water collected in the filter unit and observe the particulate material in suspension in the water.

To collect dust outfall in the open air, you can use a flat surface. To prevent the wind from blowing your sample away, this surface must be shielded by protective baffles. To accomplish this, a small plastic aquarium is suitable. To avoid collecting rain, you can place a lid at about 40 cm above the container. In any event, such a lid, you will not allow you to collect micrometeors. To do so you can observe the particles that collect in the water spout of your house.

Bear in mind that dust can also contain bacteria and other microorganisms. So, when you are handling samples of dust, whether dry or humid, you should always observe proper hygiene measures like wearing latex or rubber gloves and washing your hands after handling any potentially contaminated dust or materials.


With a stereoscopic microscope, I examined the dust I collected with a broom on the floor of my dining room. I observed grains of sand that were likely tracked into the home by shoes. There where cat hairs ( I have a cat), human hairs, textile fibers, bread crumbs and a mass of other material of unknown origin. In general, the particles were heavy and large enough to be observed under low a magnification instrument like a binocular microscope.

I then examined some dust collected on a shelf of a bookcase that was at about 2 meters above the floor. The composition of the dust observed on the shelves appears to vary, depending on the height of the shelf. Indeed, on the highest shelf, the particles are fine and apparently lighter, while those on a lower shelf are larger and appear to be heavier. As this dust is finer than the material collected using a broom, I used a normal microscope to examine it. I added the dust to a little of water and then took some drops that I placed on a slide.

To the naked eye, the dust was gray in appearance and not particularly agreeable to look at. However. when seen through the lens of a microscope, the dust appears completely different and can actually be stunning to look at. The dust I observed was composed of numerous particles and textile fibers. Many of the particles had the appearance of scales, but I could not determine if they were of biological or mineral origin. In the following pages, I present some pictures I took of this sample of dust.

To take these pictures I used an Optech Biostar B5 microscope and a Nikon Coolpix 800 digital camera with the resolution of 1600x1200 px. For the purpose of this publication, I reduced them to 400x300 px.


Figure 1 - Unstained sample of dust. 250 X

Figure 2 - Sample of dust stained with eosin. As you
can see, the most of fragments take the color of eosin. 250 X.

To distinguish between biological and mineral materials, I collected a second sample of dust and stained it by soaking it in an eosin solution for two minutes. To remove the excess eosin, I used a pipette with water and allowed some drops of water flow between the slides. This surely removed many of the finer particles but there were enough particles remaining to observe under the microscope. This staining technique demonstrated that the scaly material on my bookshelf was for the most part biological in origin. Because the bookcase is near a door that opens onto a garden, it is likely that a good part of the biological material was vegetable in origin. According researchers, a large part of the dust that collects on the furniture in a house is composed of dead skin cells and dandruff. But how do you distinguish vegetable material from animal tissue? Usually, the vegetable cell have cellular walls that are thicker than animal cells.

Figure 3 - Textile fibers. The different colour of these filaments indica-
tes their origin from different fabrics. Sample stained with eosin. 250 X.

Figure 4 - A thorny filament of unknown origin. 250 X..

Figure 5 - Unknown fragment. 250 X.

Figure 6 - Fragment of vegetable tissue. 250 X.

Figure 7 - Tuft of vegetable filaments. 250 X.

Figure 8 - Scale of a butterfly wing. 400 X.

Figure 9 - Tuft of vegetable filaments, textile fibers and
biological fragments. Sample stained with eosin. 250 X.

Figure 10 - Vegetable tuft. Sample stained with eosin. 250 X.

Figure 11 - Mushroom spores stained with eosin. 400 X.

Figure 12 - Cotton fibers after staining with eosin. 250 X.

In the samples of dust collected in the bookcase, I found different materials that required further investigation. The textile fibers are comparatively easy to identify. This is generally the case with wool, cotton and artificial fibers. In addition, several fibers of different shapes of uncertain origin were also noted. I noticed that the cotton-like fibers didn't take the colour of eosin very well or not at all. To determine if these were indeed cotton fibers, I took a little cotton batten and a fragment of optical paper, which are both composed of cellulose. I immersed both samples in eosin for two 2 minutes. After rinsing them, the staining disappeared. My observation through the microscope confirmed that the cellulose had not been stained by the eosin as shown in Figure 12.


The discussion so far has focused on dust, which consists of relatively coarse particles. These particles eventually settle out under of their own weight. There are also very tiny particles of liquids or solids, which tend to be airborne and stay suspended in the air for long periods of time. These are called aerosols. Actually, airborne dust contains extremely minute particles that are so small that you cannot even see them under microscope in addition to the particles already visible with a microscope, like the pollen. In fact most of dust is made up by particles that are coarse enough to settle out in relatively short periods of time, while the material that is much finer tends to stay suspended in the air. With the optical microscope, only the larger particles can be observed, while the smallest ones are only visible with a scanning electron microscope (SEM). The study of aerosols is actually quite an important area of interest that there is an entire branch of research dedicated to their study.

Some of the fields of study related to aerosols are as follows:

- industrial applications of the aerosols like industrial painting systems, atomizers for agriculture, cosmetics, etc.
- influence of the aerosols on the atmosphere due to particles in suspension due the human activities, forest fires and volcanoes eruptions reflect part of the sunlight and influence the climate of the Earth;
- influence of the aerosols on the human organism.

The size of the particles suspended in air is important. Our nose is able to stop the bigger particles suspended in air, but those smaller than 10 μm, called pm10 (from: "particulate matter"), are able to reach the pulmonary alveolus where the exchange oxygen/carbon dioxide occurs. The presence and the accumulation of these particles on the lungs can originate diseases.

Aerosols also occur in human environments. Fog, for example, is an aerosol. It is harmless, but some types of aerosols can be harmful to living organisms. Toxic compounds in the form of aerosols can be found in industrial environments like paint shops, steel foundries and construction sites. Outdoor environments like flowery meadows and woods will usually have airborne pollen particles and spores which may cause allergies and asthma for some sensitive individuals. Moreover, in some circumstances aerosols may contain bacteria and viruses.

Often, the indoor air quality of our homes can be affected by aerosols. Kitchens, in particular, may produce aerosols composed of extremely small particles of cooking oils and unburnt gas. Vacuum cleaners, which we use to collect dust, are used to collect larger particles, but they can also release the most minute and more harmful particles. To minimize the spreading of aerosols, the vacuum cleaners can be equipped with high efficiency filters at the outlet. These are called HEPA filters. Whenever you are sweeping or using a vacuum cleaner, it is a good practice to wear a mask. Vacuum cleaners equipped with water filters, in which the aspirated air is sparged through water, release smaller quantities of fine airborne particles. In the bedrooms where people use wool or down feather in their mattress, pillows or blankets, it likely that you will find dust mites. Dust mites produce excrements which may cause allergic reactions for certain individuals who are particularly sensitive to these allergens. Carpets may also spread dust mite excrements and fine particles when people simply walk over them.

Cigarette smoke is also an important source of unburnt airborne particles and tar. Wood-burning or kerosene stoves can release particles of this type. Fifty years ago wood-burnings stoves were commonly used for both cooking and heating. Even today, wood-burning stoves and fire places are common. Fireplaces will release smoke and can spread fine dust throughout a house. In many poorer regions, the smoke released from a hearth can accumulate in the dwelling, particularly of if the chimneys and ventilation are not adequate. While driving in a car, it is not uncommon to notice occasionally the exhaust fumes from the cars that are ahead of you. Depending on the circumstances, it is also possible for the engine exhaust fumes to leak into one's own car.


Figure 13 - View form a hillside overlooking the city of Bologna, Italy. Notice the layer of smog (arrows).

Over modern cities, you can find fine particles of soot coming from factories, motorized vehicles and electric power plants that burn fossil fuels. During the winter season, this aerosol is more prominent due to the increased activity of the power plants. When the barometric pressure drops and there is little wind, aerosols become denser and consequently more toxic. Since the end of 19th century and up tp the to the sixties, during winter the City of London was afflicted by smog (smoke and fog), formed by tiny droplets of fog that condensed around minute particles of soot and of sulphur dioxide produced by the domestic heating plants and by many factories that used coal. Because London was always immersed in smog, many people died because of respiratory problems. Some decades ago, the local government decided to correct this situation and replaced the coal with more clean-burning fuels. As a result, the smog has almost completely disappeared. The air quality in London improved and sunny days more frequent. The British people did not stop there, they decided to reclaim the Thames River also. While the work took a dozen years to complete, government agents went up the River including its tributaries with the task of eliminating or reducing the sources of pollution. As a result, the quality of the Thames has improved and fish and birds are returning to populate it. Although this has nothing to do with aerosols, it serves to illustrate how the implementation of suitable measures can help to restore the quality of our environment. As urban development progresses, many industrial cities of the world are affected by the problems of air and water pollution and the struggle to counteract this problem is becoming increasingly important.

To illustrate the how air pollution can be significant in urban areas, all you need to do is observe the colour of the mucus in your nasal passages. The colour of the mucus is indicative of the level of pollution you are exposed to. A microscopist should not have problems in dissolving a little mucus in water to observe it under the microscope. These observations will reveal organisms with cilia quickly moving about with an amoeba like movement. These are likely leukocytes that live on the walls of the mucous membranes of the nasal passage and are there to help to keep them clean.

Smokers can perform an "amusing" experiment: by placing a piece of filter paper after the filter of the cigarette to collect the particles of tar. On the filter paper, you will observe a brown stain which is produced by the particulate matter that will ultimately make there way into the lungs. Hopefully, this should convince a smoker that it is a good idea to quit smoking. Happily, many countries are promulgating laws that prohibit to smoke in closed public places. Today the people are well informed about the risks of smoking, however many people continue this habit not realising that long-term smoking can causes chronic bronchitis which can degenerate into emphysema.

While on the topic of respiratory hygiene, did you know that when you sneeze you produce an aerosol of microscopic droplets that can carry bacteria and viruses to distances of up to several meters (ten feet or more)? This is believed to be one of the means by which cold, influenza and other disease-causing microorganisms are passed on from person to person. This is why you should use a handkerchief when you sneeze in a closed space. Because of airborne microbes can infect people, medical personnel like doctors nurses and dentists use face masks to avoid contracting or spreading airborne contagions.

To reduce the amount of airborne particles of one's home, specially designed "ionizers" can be used. These devices should not be confused ozonators that produce ozone which is bad for your health. Ionizing devices produce a stream of negatively charged ions that adhere to the airborne particles nearby, causing them to deposit into surfaces or in a suitable filter.

To collect very fine particles, researchers that study aerosols use specially designed instruments that are more complex that the methods in this article.


Because its composition can vary according the location where it is collected, mere dust can be an interesting subject for observations with a microscope. The identification of the different particles contained in a sample of dust can provide a challenging and stimulating field of exploration. It is interesting that from the study of such an insignificant thing like dust, one can pursue fascinating fields of study like that of the aerosols and their impact on the climate, human health and the biosphere.

BIBLIOGRAPHY  What is Household Dust and How To Deal With It.  Mould, dust mites, fungi, spores, and pollen: Bioaerosol in the human environment.

Internet keywords: dust, dust mites, household dust, aerosol, fine particles, particulates, atmospheric particulates, particulate matter, pm10, aerosol science, smog, ionizers, allergy, asthma.

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