Microbial mishaps in the home: Discrepancies in performances of indoor dust collectors of microorganisms

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  • Published: Mar 12, 2012
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Microbial mishaps in the home: Discrepancies in performances of indoor dust collectors of microorganisms

Dusty differences

A comparison of indoor dust collectors for trapping microbes reveals vastly different performances which make it difficult to compare data from different studies. However, the collection of settled dust with one particular electrostatic device can be used as a proxy for airborne dust.

There is a common misconception that staying indoors will protect you from germs and infection but the opposite can be true, due to the fact that indoor air can be poorly circulated and refreshed. So, any microorganisms present are likely to stay there. These can include airborne bacteria and fungi, both of which can cause infection and release allergens into the air. Bacteria can also release endotoxins which are responsible for several respiratory disorders such as bronchitis and asthma.

Over the years, an assortment of different collection methods for sampling microbes have been developed to assess indoor air quality, based on electrostatic precipitation, filters, direct impact onto gels, and other methods. However, this area lacks any standardised sampling procedures, which makes it difficult to compare results collected from different devices.

A team of scientists in Denmark decided to test various sampling devices in the same indoor environment, to see how they compared. Mika Frankel, Michael Timm, Erik Wind Hansen and Anne Mette Madsen from The National Research Centre for the Working Environment and the University of Copenhagen undertook a year-long study on airborne and settled dust. Their secondary aim was to see whether settled dust, which is easier to collect, can be used as a surrogate for airborne dust that might be inhaled by the occupants.

Airborne and settled dust collection

The researchers studied the dust in five homes belonging to colleagues of the project team, who were more likely to stick to the experimental conditions for the year than unconnected members of the public. Two samplers were deployed for airborne dust collection and three methods were used for settled dust.

One of the types of airborne dust samplers was a commercial inhalable sampler (CIS) fitted with a polycarbonate filter for trapping bacteria and fungi or a Teflon filter for endotoxins. Several samplers were suspended 1.5 m above the floor in 3-4 rooms of the house for a six-hour sampling period each day.

The second airborne sampler (BioS) was a commercial glass device based on dust impingement into a solution containing an aqueous solution of sodium chloride and Tween 80. Air was drawn through both of these devices at constant rates during the collection period.

Settled dust was not collected from the floor as might be expected, to avoid potential contamination by material brought in on the inhabitants' footwear. Instead it was collected from surfaces 0.75 m above floor level, to better represent airborne exposure.

A vacuum cleaner fitted with a filter was employed during the same weeks as airborne sampling took place. In addition, cardboard boxes were positioned 1.5 m above floor level to accommodate two further methods for the continuous collection of settled dust. One set (DFC) simply held aluminium foil sheets to collect the dust as it settled from the air and the other (EDC) contained electrostatic cloths.

The dust was extracted from all of the devices for quantification of the microorganisms and endotoxins, as well as the measurement of total inflammatory potential (TIP) by a chemiluminometric assay based on differentiated HL60 cells.

Electrostatic advantages

Of the two airborne dust samplers, the CIS device consistently gave higher estimates of the levels of microbial agents trapped than the BioS device. The researchers expected the liquid environment of the BioS to be more conducive to survival and growth of bacteria than the dry filter of the CIS device, but the opposite occurred in practice.

Despite this, the two sets of data correlated strongly, suggesting a systematic difference in the way that the samplers operated. The lower sampling efficiency of the BioS was attributed to the possible re-aerosolisation of the particles. Remarkably, the TIP values of the BioS sampler were higher than those of the CIS device, which was attributed to interference in the assay from Tween.

For settled dust, the levels of all the microbial agents and the TIP values were far greater for the EDC than the DFC device. The difference might be due to the easy escape of particles from the smooth surface of the DFC sampler and the fact that the collected dust must be vacuumed from the surface onto a filter, resulting in further losses. Nevertheless, the two methods showed a strong correlation for all the microbial agents, indicating that the proportions of the various bioagents were similar.

The results from the third method which involved vacuuming open surfaces correlated well with the EDC and moderately with the DFC.

When the results for airborne and settled dust were compared, those for the EDC device were the most representative of airborne dust and showed a strong correlation. So, settled dust collected with the EDC could be used as a surrogate for inhalable microbial exposure, which would facilitate studies on the assessment of indoor exposure to microbes.

As it happens, the EDC is the easiest of the three settled dust collectors to use, can be mailed to participating homes, and the electrostatic cloths can be extracted directly for analysis. It also represents exposure over a longer period than a six-hour airborne collection period, averaging out day-to-day variations in the indoor microbial content.

The results can "be used as a frame of reference when studying the literature or when conducting further studies on indoor microbial exposure," concluded the research team.

Related Links

Indoor Air 2012 (Article in Press): "Comparison of sampling methods for the assessment of indoor microbial exposure"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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