Air Pollution by Aluminum Compounds Resulting from Corrosion of Air Conditioners Dmytro Buchnea*
Banting and Best Department of Medical Research, University of Toronto Alexander Buchnea
Ecolex Ltd., Toronto, Ont., Canada Hydrated aluminum oxide (A1203.H20 and A1203.3HzO), chlorinated aluminum oxide (A1203*HCl), and carbonated aluminum oxide (A1203.COz) contribute to air pollution as products of the corrosion of air conditioners. In cold rooms a t temperatures of 3-6"C, this pollutant precipitates from the air coating all surfaces in the rooms with a film of gray dust. In air-conditioned rooms at normal temperatures of 20-25"C, this aluminum-based film was not evident; however, the air contained approximately 10 times as much of this pollutant as that in the cold rooms. This concentration of dust corresponds to an unacceptably high concentration by the Air Quality Standards of the Government of Ontario. During the summer (when the air conditioner was a t peak operation) the air contained more of this pollutant than during the winter (when the air conditioner was not operational). Daytime and nighttime concentrations of dust in air-conditioned rooms during the summer also differed appreciably. W
Properties of Aluminum. Aluminum is a widely used material because of its high thermal and electrical conductivity, its light weight, and its resistance to corrosion. This resistance to corrosion results from the properties of the thin transparent coating (about 6.4 x 10-6 mm thick) of aluminum oxide which covers all aluminum surfaces ( I ) . Its high density, high melting point, and great isolubility make it very unreactive under normal conditions. However, mineral acids or strong alkalies will react with it (2, 3 ) . All of the above are reasons for the extensive use of aluminum filters and grills in air conditioning units. Corrosion of Aluminum Parts of Air Conditioners. The following investigation was motivated upon discovery of a thick layer of gray dust covering all surfaces in various cold rooms, the air of which was maintained a t a temperature of 3-6°C by air conditioning units during both the summer and the winter. The dust was analyzed and proved to be an aluminum-based compound (Table I). Details of the analysis follow in the next section of this paper. Since all the air (from which the dust would precipitate) in the cold rooms must pass through the air conditioner unit, this was the natural source to suspect. Upon visual examination, the aluminum grills of the air conditioners were seen to be severely corroded (Figure 1).The corroded layer had an identical composition to that of the gray dust collected in cold rooms (Table 11). Additional evidence of the corrosion of air conditioners came recently when the authors received aluminum grills from air conditioning units installed in a hospital (about 100 miles from Toronto) (Figure 2 ) . The composition of the corroded layer is given in Table 111. With this evidence of visible corrosion on the aluminum grills of air conditioning units and the precipitation of the same dust from the air on surfaces in cold rooms, the 752
Environmental Science & Technology
question arises as to how much aluminum dust is present in the air itself. It would also be interesting to compare this with the concentration of aluminum dust in air-conditioned rooms at normal temperatures of 20-25°C where negligible quantities of surface dust were evident. To answer the above questions, the following five experiments were carried out: The dust concentration of the air in three cold rooms (3-6°C) was measured and the percentage of aluminum compounds was determined. The low temperature was maintained by air conditioners. The dust concentration in air of an air-conditioned office a t normal temperature (20-25°C) was measured, and the percentage of aluminum compounds was determined. This office was connected to a central air conditioning system through which the air was passed whether or not the air conditioner was operating. The measurements were made both in summer(s) when the air conditioner was a t peak operation, and in winter(w) when the air conditioner was not in operation. The dust concentration of the air in an office at normal temperature with no air conditioner was measured and the percentage of aluminum compounds determined both in summer(s) and in winter(w). (a) The dust concentration of the air coming directly from a single air conditioner was measured and the percentage of aluminum compounds determined. ( b ) The dust concentration of the air in the same room was measured with the air conditioner sealed off and not in operation for a period of at least 24 hr before the measurement. The dust concentration of the outside air was measured and the percentage of aluminum compounds determined. All of the above rooms were in the same immediate vicinity and, apart from the air conditioners and temperature, can be taken to have identical environments. Sampling periods were for 12 hr, and separate measurements were made day and night. Both summer and winter experiments were conducted consecutively within the shortest period of time possible, and thus, there is no reason to anticipate a change in inside atmospheric dust composition for the different measurements.
M e t h o d s of Chemical Analysis The dust was examined under a microscope and appeared as a conglomerate of crystalline granules (Figure 3). This dust was insoluble in water, but dissolved in 5N mineral acids (hydrochloric, nitric, or sulfuric acid), in most cases, with evolution of gaseous carbon dioxide. The chemical composition of the dust is given in Table I . This was determined as described below. Dusts from three cold rooms and the corroded layers from two different aluminum grills were analyzed separately. The percentage of hydrated aluminum oxide, chlorinated aluminum oxide, and carbonated aluminum oxide was calculated from the amount of aluminum, hydrated water, chlorine, and carbon dioxide found in the dust (all free surface water having been removed). since other inor-
ganic matter was present only in a negligible amount (Tables 1-1:
Dete
..._ I . r d r ~Y I tiv!ww!\=r unit from cold room of 3 % installed about 20 years ago. In upper corner left,enlarged sec.. . , . , ~. .,, -:
rngurr
_.__-
the SOL content water. LL,c buLILt.III auilaGc waLc.I W n a ycLc.Imined by drying the dust samples a t 110-115”C, until a constant weight of the samples was reached (ahont 24 hr). The loss in weight (Le., the weight of free water) varied with the origin of the dust sample. For example, the surface dust samples from the cold rooms a t 3-6°C contained on the average 27.3090 of water. The dust sample from the corroded layer of the air conditioner operating constantly a t YC, had a free water content of 34.5090. The high water content is due to the high precipitation of atmospheric water vapor within-the air conditioner, which condenses around the nucleus of dust. The dust sample obtained from the corroded layer of an air conditioner operating a t a normal room temperature (20-25C), however, had a free water content of only 14.58%. (b) The dry dust samples were then calcinated until a constant weight was reached to determine the amount of hydrated water. The loss of hydrated water varied from 37-4590, which indicated that the hydrated aluminum oxide is present as a mixture of diaspore (A1203.HzO) and gihhsite (Alz03.3H20) in varying proportions (4). The amount of aluminum oxide (A1203) remaining after calcination was in good agreement with the amount of aluminum obtained by precipitation as AI(C$HeON)3 (as described below). Determination of Aluminum in Dust. All the dust samples were analyzed separately. The dust was first dissolved in diluted hydrochloric acid and the solution was cleared by filtration. Then on concentration to dryness, the quantitatively recovered aluminum chloridq was dissolved in water made very slightly acidic with hydrochlo-
Table I. Average Composition of Gray Dust Collected from Three Air-conditioned, Cold Rooms (34°C) 7,
Water
Figure 2. Grill from air conditioning system installed in hospital two years ago. In upper corner left, enlarged Section of corroded aluminum grill
n,ararco d u m n L m axme ( A I 0 34 0) C’I orinatro illmi TI Jm ox de ( A O..nCI) Cituormted A d m n JII ox oe [A 0 . C O )
Organic matter Unidentified inorganic matter
?i.w GI 55 J .20 2.11
1.62 0.68
Table II. Chemical Composition of Corroded Layer of Aluminum Grill of Air Conditioner from Cold Room of 3”C, Installed About 20 Years Ago (Figure 1)
Water Hydrated aluminum oxide (Ai20r.3Hz0) Chlorinated aluminum oxide (AI$Oa.HCI) Carbonated aluminum oxide (Ai,O,.COz) Organic matter Unidentified inorganic matter
34.50 54.40 4.40 4.50 1.30
0.30
Table 111. Chemical Composition of Corroded Layer of Aluminum Grill of Air Conditioning System from Hospital, Installed About Two Years Ago (Figure 2) ?& ..
Figure 3. Dust particles, magnification~200X.of hydrated. chiorinated. and carbonated aluminum oxide appearing as conglomerate of crystalline granules
Water Hydrated aluminum oxide (AI,0,.3HIO) Chlorinated aluminum oxide (AI,O,. HCI) Organic matter Unidentified inorganic matter
14.58 75.90 7.42 1.40 0.70
Volume 8 , Number 8. August 1974 753
, ric acid. Then the aluminum was precipitated with 5% solution of 8-hydroxyquinoline in 2N acetic acid as Al(CgH60N)s ( 5 ) . The precipitate, after filtration and drying at 130°C,was weighed and the weight of aluminum was calculated. The weight was converted into the corresponding weight of A1203. Then according to the amount of hydrated water, chlorine, and carbon dioxide measured using the techniques described above and below, the percentages of the respective aluminum compounds were determined (Tables 1-111). Determination of Chlorine. Each sample of dust was first dissolved in nitric acid, and the solution was cleared by filtration. The chlorine was then quantitatively precipitated with 1N silver nitrate solution a s silver chloride (AgC1). The amount of chlorinated aluminum oxide (Ah03.HCl) was determined from the amount of chlorine (Tables 1-111). Determination of Carbon Dioxide. The amount of carbon dioxide in the dust samples was determined by a direct method as described by Kolthoff and Sandell ( 5 ) .The carbon dioxide from each sample was completely liberated with 5 N hydrochloric acid as gaseous carbon dioxide and absorbed quantitatively on Ascarite (sodium hydroxide asbestos) and was then determined by weight. The percentage of the carbonated aluminum oxide (A1203.C02) was then calculated according to the amount of C02 in the dust samples (Tables 1-111). Determination of Air-Borne Aluminum Compounds. The determination of aluminum in airborne dust was carried out as follows: T o obtain greater accuracy the dayand nighttime samples were combined and the filters were extracted with diluted hydrochloric acid. After filtration and concentration to dryness, the residue was dissolved in water. The aqueous solution of aluminum chloride was then precipitated with 8-hydroxyquinoline solution into Al(CgHsON)3, as described above. The precipitate was then converted by calculation into the respective aluminum compounds according to the amount of aluminum found in airborne dust as described above (Table IV). Since the airborne dust collected from air-conditioned rooms has similar physical and chemical properties as the gray dust obtained from cold rooms and as that of corroded layers, it was anticipated that the aluminum com-
Table IV. Average Volume of Filtered Air: 40 m3 During the Day and 40 m3 During the Night Dust concentration was gravimetrically measured and percentage of aluminum compounds determined. Following results were obtained from experiments described in text. Dust collection time: During the day (12 hr), and during the night (12 hr). In summer (s) August, and in winter (w) December
Dust concn, rg/m3
Expt no.
1. 2.
3.
4.
5.
754
Conditioned air a t 3" to 6°C Conditioned air i t 20" to 25"
Day
Ninht
com-
pounds in dust, %
152 i 9 7 i 0.4 64 i 4 364 + 9 34 & 0 . 7 175 i 5 27 =c 4.0
No air conditioner normal temperature Airdirectfrornairconditioner Air with air condiditioner sealed off Outside air measurements
Concentration of aluminum
1 i 0.1
0
+ 0.1
Night
49
Day
59 i 5 36 i 3.6
kiht
49 =t5 32 =t4 24 i 5 42 i 5
Night
Environmental Science & Technology
I5
2 i 0.1
.O i 0.1
pounds in the airborne dust also would have a similar composition.
Experimental Method and Results Airborne Dust Collection Experiment. The concentration of the dust in the air was initially monitored using a Beta-Gauge instrument for airborne dust concentration measurement developed by one of the authors (A. B.) for Ecolex Ltd., Toronto. The dust monitoring instrument works on the principle of beta absorption in matter (6, 7). A predetermined volume of air is passed through a filter paper (Pallflex No. E70-2075W) capable of trapping particles down to 0.5 p with 99% efficiency. Beta rays (maximum energy = 0.156 MeV) from I4C are transmitted through the filter paper and are counted before and after dust collection. The difference in counts, statistically significant for concentrations of dust above 20 pg/m3 in the atmosphere, can be directly related to the weight of dust on the filter paper. This value, together with the measured volume of air passed through the filter, can be related to the concentration of the dust in the air. The electronics of the machine were set for only a onehalf hour cycle time. Since this period of time was insufficient for collecting an adequate amount of dust for chemical analysis, quantitative measurements were made gravimetrically with a preweighed piece of filter paper following a 12-hr suction period during which time approximately 40 m3 of air passed through the filter paper. The integrated volume of air was measured using a diaphragm gas meter (8) calibrated for air and having a built in automatic temperature compensator. The suction system was composed of a pump, meter, filter paper (with approximately 6.5 cm2 cross section through which air passed) and an inlet. Between the inlet and the pump, the system was completely closed so that only the air passing through the filter paper was recorded in the meter. The filter paper was weighed (after drying for 1 hr in a desiccator) before and after deposition of dust, and from this the concentration of the dust in the air was determined. Experimental Results of Airborne Dust Analysis. The results of all experiments done in (i) air-conditioned rooms a t low temperatures (3"-6"C), (ii) air-conditioned room a t normal temperatures (20°C to 25"C), [(s) August results] and [(w) December results], (iii, room with no air-conditioner a t normal temperatures [(s) August results] and [(w) December results], (iv) dust collected direct 'from a single air-conditioner unit [results ( a ) ] and the same room with the air-conditioner sealed off [results (b)], and finally the dust concentration of the outside air [results (v)]are presented in Table IV. From Table IV the following results become obvious: (1) Aluminum compounds constitute a major proportion of the dust found in air-conditioned rooms at normal temperature. (2) In cold rooms, the percentage of aluminum compounds in the air is still significant but much lower than in air-conditioned rooms a t normal temperature. This was to be expected since such a large amount of the aluminum . compounds was precipitated as surface dust. (3) Both in the office without an air conditioner and in the outside air, the percentage of aluminum compounds in the air was either negligible or very small. As the office with and without air conditioning units had in effect identical environments with the exception of the air conditioner, the aluminum compounds may be seen to come directly from air conditioning units.
(4) Experiment 4 was performed as a direct test of the above. Air coming directly from the air conditioning unit contained 36% aluminum compounds. This is comparable to the percentage of aluminum compounds in air-conditioned rooms (Experiment 2) a t a normal temperature where a state of equilibrium must have been reached. Furthermore, upon the sealing off of the air conditioning unit, the concentration of aluminum compounds in the air of this room dropped to a nearly negligible value. In the air-conditioned room (Experiment 2 ) , inasmuch as the air conditioning system was a central one, the aluminum dust had permeated throughout the system so that although the total dust concentration decreased considerably, in winter when the air conditioner was not in operation, the percentage of aluminum compounds remained almost the same. The above results clearly indicate that the air conditioning units are a major source of air pollution. This is vividly illustrated by the great decrease in the total dust concentration in the air-conditioned office from summer [Experiment 2 (s)] to winter [Experiment 2 (w)] measurements, whereas that in the office with no air conditioner remained about the same. In summer the total concentration of dust during the night in air-conditioned rooms dropped to one half of the value during the day. As the air conditioners are not operational as much at night as in the day, this difference is to be anticipated. Since day and night dust samples were combined to improve the accuracy of the chemical analysis, no comparison of the percentage of aluminum compounds during the day and the night could be made.
Conclusions During summer days, when the air conditioner was a t peak operation, there was a dust concentration of 364 pg/m3 in an air-conditioned office. This dust contained approximately 124 p g of aluminum-based compounds in 1m3. A concentration in dust of 365 pg/m3 of air corresponds to a dust concentration level unacceptable by Ontario Air Pollution Standards (9). A person inhales air at a average rate of 0.116 x m3/sec (10); thus, during a 10-hr period in an air-conditioned environment, 4.2 m3 of air are inhaled in which there are 513 pg of aluminum-based compounds. Only those particles whose diameters are less than 1 p reach the aveoli of the lungs and of these, maximum retention occurs a t diameters between 1.0 and 0.5 p but below 0.2 p ( 1 1 ) . This retention depends also on the rate and depth of respiration. Since in this particular case a size distribution is not known, 1 he rate of accumulation of aluminum compounds is not known nor the physiological effect of such an accumulation over a long period of time. The different behavior of the dust in cold rooms in which there is a large amount of surface dust and a relatively low concentration in the air, compared with the reverse situation in normal temperature rooms, is related to the
percentage of free water in the dust. The percentage of free water in the dust of cold rooms is twice that in normal-temperature rooms. Thus, the dust acts as a nucleus around which water vapor condenses. The amount of condensation is much greater a t a cold temperature than a t a warm temperature and thus larger particles are formed due to the increase in water content and the rate of precipitation from the air is greatly increased. This results in a low concentration of dust in the air. We tried to explain the phenomenon of the reaction of atmospheric water vapor and carbon dioxide with aluminum oxide coating as follows: It could be possible that the air contains a pollutant, for example chlorine, which catalyzes atmospheric water vapor and carbon dioxide, and they then react with A1203 to produce A1203.H20, A1203.3H20, A1203.CO2, and A1203.HC1, respectively. If this is so, then all aluminum appliances would be corroded. But, it seems that the corrosion is restricted only to the air conditioners. Therefore, it seems that the air circulation mechanism characteristic to the air conditioner is involved in the activation of the atmospheric water vapor and carbon dioxide, which react with the coating of aluminum oxide, and thus corroding the aluminum parts of the air conditioner. The particles of this corroded layer are then dislocated and diffused by air circulation into the air of the air-conditioned rooms. This hypothesis is borne out by the drastic decrease in the dust concentration when the air conditioner is no longer in operation.
Acknouledgment We thank E . Wood, administrative coordinator of Banting and Best Department of Medical Research, for initiating this investigation and R. Hubbard for technical assistance.
Literature Cited (1) Encyclopaedia Britannica, Vol. 1, p 715, col. 1, 1960. ( 2 ) Chammond, C. R., (CRC) Handbook of Chemistry and Physics, 51st ed.. p B-5, 1970-71. (31 Parker, R. H., “An Introduction to Chemical Metallurgy,” p 326. 1967. (4) Partington, J. R.. “Textbook of Inorganic Chemistry,” 6th ed. p 805, 1953. (5) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis.” 3rd ed., pp 318 and 370. Macmillan Co., New York, S . Y . , 1952. (6) Aurand, K . , Bosch, J., S t a u b , 27 445 (1967). ( 7 ) Dresin, H., Fischotter, P.. Felden, G., VDI-2, 106, 1191 (1964). (8)Canadian Meter Co. Ltd., SIeter No. ALC 175. (9) Ontario Air Management Branch, Government of Ontario. private communication. (10) Dienhart, C. M..“Basic Human Anatomy and Physiology,” pp 151-2, W . B . Saunders Co., Philadelphia, P a . , and Toronto, Canada, 1973. (11) Air Pollution, World Health Organization, Geneva. 1961.
Receiced for rerieri M a r c h 5 , 1973. Accepted April 22, 1974. Work supported b> t h e Health Sciences C o m m i t t e e , Cnicersity of Toronto, and Ecolex L t d . , Toronto.
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