A simple method for the measurement of dust on surfaces and the effectiveness of cleaning

A simple method for the measurement of dust on surfaces and the effectiveness of cleaning
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  Environment International, Vol. 15, pp. 563 - 566, 1989 0160-4120/89 3.00 +.00 Printed in the U.S.A. All fights reserved. Copyright @1989 Pergamon Press pl SIMPLE METHOD FOR THE ME SUREMENT OF DUST ON SURF CES ND THE EFFECTIVENESS OF CLE NING T. Schneider O. Holm Petersen P. Eriksen and P. Vinzents National Institute of Occupational Health Copenhagen Denmark B. Kofoed Hansen Institute of Cleaning Technology Taastrup Denmark E1 87-289 Received 26 October 1987; Accepted 12 January 1989) An objective method for the assessment of the effectiveness of cleaning routines has been developed. The surface contamination is first sampled by sticky gelatin foil. The amount of dust, the dust index, is then determined by measuring the light which is diffusely scattered through the foil, and calibrating it against the area covered by dust, in percent, as determined by microscopy. The method is reproducible, easy to use, and a dust-covered area of less than 0.1% can be detected. A sampling strategy was formulated and used to demonstrate the effectiveness of a weekly cleaning routine. Objective methods, such as the one described here, can be used to collect data in a systematic way to provide a baseline indicating how low-dust levels can be achieved. Hopefully, this will lead to the setting of objectively based cleaning standards. INTRODUCTION The most important task of a cleaning program is to remove dirt and dust from surfaces. The result is usually assessed by a walk-through visual inspec- tion. A visual evaluation of the amount of dirt and dust present on a surface is dependent upon the color, the roughness, and the illumination of the surface as well as upon individual perception. The degree of surface contamination and the effectiveness of clean- ing cannot be assessed objectively by visual inspec- tion alone. Therefore, methods that allow for objective measurements of dirt and dust are needed. Such meth- ods can also contribute to make cleaning programs cost-effective. A variety of measuring principles have been pro- posed to assess dust accumulation and surface contamination. Collection of dust and airborne mi- croorganisms by sedimentation onto Petri dishes are well established techniques, but have their shortcom- ings. Air velocity, direction and turbulence affect deposition, and edges cause shadowing Solomon 1984). Recently, a transparent plastic foil covered with a polyacrylic binder has been described for oatdoor use Schultz and Ober 1986). Such samplers placed on indoor surfaces can interfere with the de- position, resuspension, and redistribution process, a problem which can only partly be overcome by the surface test method Anzai and Kikuchi 1978). The wipe test is widely used in industrial hygiene OSHA 1977) and consists of wiping off a predetermined surface area with a wetted filter paper. Since resus- pension factors can vary several orders of magnitude 56  564 T Schneider et al (Sansone 1987) surface sampling cannot accurately evaluate the air contamination, although several tests simulating resuspension have been described. The purpose of cleaning is not only to remove dust which can be resuspended but also to clean the sur- face, as such. It was therefore decided to develop a method which sampled a well defined surface area, was reproducible, required a minimum of sample handling, and where the analysis was quick and easy. The method should provide the surface contamina- tion at any time, without preparations, as well as the fraction of contamination removed by a cleaning pro- cedure. By thoroughly cleaning a surface, the dust accumulation on that surface over a time interval T can be established by sampling at time T after the thorough cleaning. This procedure will not interfere with the accumulation process. METHOD OF ME SUREMENT Special sticky foils, so-called footprint lifters, have been developed for forensic purposes. These foils can follow slightly irregular surfaces and have good sticking properties. They have been used with excel- lent results in detection of mineral wool fibers in the indoor environment (Schneider 1986). Since the foil surface is very smooth even after sampling, and since the foil has excellent optical properties, it is possible to detect the presence of particles on the foil by light scattering. The light scattering principle has been used for quantifying the amount of dust deposited onto glass slides (Broomhead et al. 1960). The adopted method is described in the following paragraphs. Footprint lifters are cut to a suitable size. The top cover sheet of the foot-print lifter is removed from about 2/3 of the sticky foil area which is then pressed towards the surface by hand. After the foil has been lifted off with the sample of dust, the top cover sheet is gently rolled back in place. In the laboratory, a circular sample (12 mm diameter) is cut from the foil (top cover sheet removed, bottom cover sheet left in place) and placed in a holder (Fig. 1). Aparallel beam of light from a halogen lamp is directed normally onto the sample. The total transmitted light, I o, (or total transmittance) is measured with an integrating sphere (ClE 1986) using a silicon-photodiode with a photopie filter as a detector. When the non-deflected, transmitted light is allowed to escape the sphere (regular or direct transmittance), the diffusely trans- mitted light, I r, is measured. All values are corrected by subtracting readings from measurement of a blank (unused part of the same foil). The percent of dif- fusely transmitted light DT=100 IT/I o, is used as the index of surface contamination. Particles on a surface will tend to be unevenly distributed. The inter-foil variation in DT for a given surface, such as a desk or a shelf, could be described by a constant coefficient of variation (CV). CV de- pends on the measuring situation and was found to range from 20 to 45 . There is a tendency for CV to become smaller for larger values of DT. The dust index was calibrated against the area A (in percent) covered with dust particles. Area A was determined from a different area of the same foil by optical microscopy (with a microscope projection attachment and an interactive image analyzer). A linear regression of the square-root-transformed variables gave the following equation (see also Fig. 2 ). DT -- 2.36 A (95 confidence interval 2.00-2.76). The correlation coefficient was r = 0.95. EXAMPLE OF APPLICATION The dust index was determined before and after the cleaning of desks in classrooms in two schools. A two-way analysis of variance assuming a linear model Integrating sph~ portJ ~port Foil with du~ port 2T Si photodiode with photopic ilter Fig 1 Principle of analysis using light scattering into an integrating sphere  Measurement of dust 565 6 00 (% Diffuse transmittance, DT)0 5 5.00 .~ 4.00 . .~ 300 . . ,* DT -- 2.36A 2.00 1.00 . . ° • I I , I , I t 0.00 1.00 2.00 3.00 4.00 (% Area covered, A) 0 5 Fig. 2. Plot of diffuse transmittance (%) vs area (%) covered with dust (square-root-transformed variables). c E c O @ 1 0 0 5 desk • 41 ..a_ | T 1 2 3 4 2 1: First monday after cleaning shelf L • • I I i I 3 4 3: Second monday before cleaning desk _a_ a I l I I I I 1 2 3 4 1 2. Friday after work shelf .. - - i . I 234 4: Second monday after cleaning Fig. 3. Dust on surfaces expressed as % diffuse transmittance for a desk and shelf from room A and B, followed over a week. Results from single positions (,) and surface area averages (--). showed a significant (p < 0.05) reduction of the dust index after cleaning. This demonstrates that the dust index is a good descriptor of the state of cleanliness in the rooms. In another investigation, the surface contamina- tion of a desk and a shelf was monitored four times in two office rooms over an entire week; starting Monday morning after the weekly cleaning was done, but before work started (1 on Fig. 3), Friday after work (2), Monday before cleaning (3), and after cleaning (4). The desk and shelf surfaces were divided into a series of numbered areas. For each sample, the sam- piing position was selected according to a table of random numbers, precluding multiple sampling• Each day 5 samples were taken from the desk and shelf, respectively. The cleaning program consisted in daily wiping of ~he desks. Shelves were wiped Mondays and Wednesdays. Fig. 3 shows single and average values. Samples which, according to the sampling scheme, happened to be from underneath a letter tray, had high values of DT (1 to 11 ) and were omitted. It was supposed that cleaning had not reached these areas. Two samples had outlying blank values and were omitted. The increase in surface contami- nation from Monday to Friday is apparent from Fig. 3, as is the effect of cleaning on the second Monday. Values of the dust index exceeding 2 are common, and so it is encouraging that even with such low values, as shown in Fig. 3, the method gives consis- tent results. This is demonstrated by the fact that there was no statistically significant difference be- tween the two Mondays after cleaning. It turned out later that room A had been cleaned before samples on the second Monday (before cleaning) were taken.  566 T. Schneider etal. iffuse transmittance ABOVE 30.0 2v.o 3o.o ='-- 24.0 - 27.0 '==~= 21.0 -- 24.0 1S.O -- 21.0 15.0 - 18.0 12.0- 15.0 9.0- 12.0 ~ 6.0 9.0 3.0 - 6.0 I~[ BELOW 3.0 Fig. 4. Mapping of surface contamination on the floor and on the tables and shelves in an office. The measurements were made with a preliminary version of an improved optical system and the absolute values are not directly comparable to those in Fig. 3. This can be seen from Fig. 3. The measurements can thus detect that a given cleaning program was not followed in practice. Since a large number of samples can be taken and analyzed in a short time, rather extensive mappings of the surface contamination are possible. An exam- ple is shown in Fig. 4. The plot was made with the GEOPAK 1984) interpolation and plotting program. The visualization of the dust on the surfaces by this mapping technique is very instructive and can be used for training cleaning personnel. DISCUSSION ND CONCLUSION Surface sampling by footprint lifters and subsequent determination of sampled dust by light scattering has been shown to be a quick simple and reproducible method for evaluating the general dust contamination of non-textile surfaces. Practical applications include assessment of the effectiveness of cleaning programs and characterization of surface contamination pat- terns in buildings. Continued data collection may provide a baseline indicating how low a level of surface contamination one can achieve and maintain in various room and building types. Hopefully this will lead to the setting of objectively based cleaning standards. The contamination of carpets cannot be measured directly, ut dust redispersed from carpets has a great probability of settling onto surfaces which can then be sampled with the sticky foils. The method will therefore reflect the dust emission from textile sur- faces. A much improved optical system is now being developed which is much more accurate and which requires even less sample preparation. REFERENCES Anzai, I. and Kikuchi, T. A new monitoring technique of surface contamination - the test surface method. Health Phys. 34, 271- 273; 1978. Broomhead, G.0 Hodkinson, J.R., and Simons, V. Photometrische Auswertung yon Konimetrischen und anderen Staubproben. Staub 20, 144-150; 1960. Commission Intemationale de 1'Eclairage, Measurements of re- flectance and transmittance I. draft; Occupational Safety and Health Administration, CIE, Berlin; May 1986. GEOPAK Users manual UNIRAS European Software Contrac- tors, Lyngby, Denmark; 1984. Occupational Safety and Health Administration, Wipe sampling policies and procedures Industrial Hygiene Manual, 115-116. OSHA, Washington DC; 1977. Sansone, E.B. Redispersion of indoor surface contamination and its implications. In: Treatise on clean surface technology Mit- tal, K.L. ed., 261-290, Plenum, Vol. I; 1987. Schneider, I . Man-made mineral fibers and other fibers in the air and in settled dust. Environ. Int. 12, 61-65; 1986. Schultz, E. and Ober, W. Application of image analysis in particle deposition measurements. In: Aerosol Formation and Reactiv- ity, 817-820. 2nd. Aerosol Conference Berlin, 1986. Pergamon Press, Oxford, OK; 1986. Solomon, W.R. Sampling techniques for airborne fungi. In: Mould allergy. AU-Doory Y. and Domson J.F., eds., 41-65, Lea & Febiger Philadelphia PA; 1984
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