World Trade Center Dust: Its Potential to Interact with Artifacts & Works of Art
Mary Ballard, Senior Textiles Conservator and Member of the AIC Health & Safety Committee

Dust residues from the collapse of the World Trade Center (WTC) buildings may carry a mix of components not normally encountered with urban street dust. Currently the Environmental Protection Agency (EPA) [4], the New York Committee for Occupational Safety and Health (NYCOSH) [6], Occupational Safety and Health Administration (OSHA) [8] are focusing on potential toxic contaminants in the air, on health hazards, and on suitable protocols for cleanup. This review focuses on the characterization of the dust, its potential to interact with artifacts and works of art in the vicinity, and the possible contamination of the conservation studio.

Dust refers to the mixture of airborne materials that settle upon surfaces. These airborne materials may have organic components but are most often inorganic in character. Because of variations in density, micron size, air currents, wall barriers, and filtration methods, the exact quantitative ratio of one component to another will not necessarily be consistent across a room, throughout a building, nor in a neighborhood [11]. While the chemical interaction of WTC dust with artwork is possible with certain dust components, the physical interaction (abrasion) is an important feature of small and hard materials. The EPA is monitoring small and large particulates, dividing them into two category sizes: under 10 microns and over 10 microns. The air quality index is based on the 24 hour accumulation of fine particles: more than 40 micrograms (millionths of a gram) in a cubic meter of air [mg/m³] is considered harmful to sensitive individuals.

No differentiation is being made among the >non-hazardous= particulates for chemical composition, crystalline structure, or size [4]. Of particular interest to conservators would be the smallest (less than 2.5 micron) particles since they are lightest and most difficult to control in a work space or studio[4]. They are generally too small for identification by conventional light microscopy. A precise analysis of dust components on a particular object can be carried out by commercial chemical analysis. Qualitatively, the components of the WTC dust residues are known to include the following materials:

Asbestos, in one of its forms, is a naturally-occurring, mineral fiber composed of hydrated magnesium silicate, Mg₃(Si₂O₅)(OH)₄ [5]. It is chemically inert up to 500^o C: it does not burn (combust)[9]. The chrysotile fiber form was widely used in building construction for its fire retarding and heat insulating properties until it was determined to be carcinogenic and hazardous to the lungs. Optical light microscopy is insufficient for a positive identification of the fine fibers, as they may measure only 0.5 micron in width [10]. The chrysotile form reacts to heat in a two stage manner: in the range of 600-780^o C, the compound dehydrates (looses water). At 800-850^o C, the anhydride breaks down into forsterite and silica. Chrysotile asbestos has a Mohs hardness of 2.5-4.0 [5].

Using phase contrast microscopy upon bulk samples on debris and rubble at the WTC site 9/13-9/27, OSHA found asbestos contents ranging from non-detectable to 1.9 fibers/cubic centimeter (f/cc) with 50% of the asbestos found in the chrysotile form [8]. Settled dusts containing 1% measurable asbestos are listed as regulated material for handling and disposal under federal and state laws [4, 6]. Test samples by the EPA vary from non-detected to near 5% asbestos fiber [6, 4].

Silica and silicates occur in many forms. Synthetic vitreous fibers (SVF) or Afiberglass@ are widely used for insulation and have a presence at the site[8]. When these staple glass fibers have dusts less than 3.5 microns (diameter) x 10 microns, they are considered a health hazard [5, 8]. Voluntary OSHA exposure limits are 1 f/cc during an 8 hour period. Specific monitoring of airborne silica by OSHA at the WTC site found ranges from twice the OSHA limit to less than detectable quantities [8]. Another amorphous form of material largely composed of silica is window glass. In this instance, it would be present as a silica dust, also monitored.

Other crystalline silicate powders normally occur in cities as buildings weather. The Acanyons of Wall Street@ and, Manhattan generally, have large quantities of cement and concrete. As a construction material, concrete is a mixture of portland cement, sand, gravel, and crushed stone, reinforced with steel rods and mesh [2]. Portland cement is a hydraulic cement: water activated, set, and hardened into non-water soluble material, containing the hydrated forms of tricalcium silicate (3CaO×SiO₂) and dicalcium silicate (2CaO×SiO₂) [5]. Thus, non-fibrous forms of silicates and other inorganic minerals are typically present in urban dust. Concrete, cement, and mortar dusts, become alkaline when moisture is present; materials susceptible to alkaline degradation or alkaline-triggered reactions could be affected. Some dust samples are showing 30-60% silicates [1]. On the Mohs scale, glasses and polycrystalline silicates lie between 5 and 7, and thus are capable of scratching copper, iron, aluminum, mild steel, and marble [5].

Air pollutants OSHA has found trace levels of nitric oxide and nitrogen dioxide at the WTC site; sulfur dioxide levels were also below OSHA limits [8]. These are reactive gases that may combine with particulates and also affect the pH of the settled dust, making it acidic when combined with moisture. These gases are typically found as components of urban atmospheres.

Organic compounds. Volatile organic compounds, emanating from the fires burning on the rubble, have been monitored by OSHA at the WTC site. Dioxins were also found at the site as were trace amounts of polychlorinatedbiphenyls (PCB=s) [8]. Again, these are ubiquitous in urban environments. The EPA is monitoring the smoke plumes at the WTC site for these materials [4]. Other organic solvents used in air conditioning systems, like Freon R-22, and their thermal decomposition products, like hydrofluoric acid and phosgene, have not been found at detectable levels at the site by OSHA [8].

Other problems. Similarly no ionizing radiation has been detected at the site. OSHA has monitored for latent radiation with attention to a-radiation [8]. Metal dusts, including those of cadmium, copper, iron oxide, lead, arsenic, and mercury, are being monitored by OSHA at the WTC site[8].

Summary Unlike typical soot or urban dust, soiling upon objects as a result of the World Trade Center=s collapse will contain the Asettling@ of dust with levels of abrasive materials, possibly injurious to conservators and to objects, and additionally subject to hazardous waste procedures. This siliceous dust is particularly hard and potentially sharp. It can scratch bone and horn, bronze and marble, as well as cut yarns and fibers. Painted and coated surfaces are not immune from this damage. Under these circumstances, the disturbance or spreading of the dust in the work space should be avoided, restricted. Special attention must be paid to preventing contamination. While conservators are working, personal protective equipment is recommended. Safety procedures are essential for a successful treatment program (see AWorld Trade Center Dust, Safe Work Practices For Conservators@).

In order to prevent the WTC dust from damaging artwork, special care must be taken to avoid sweeping, brushing, or other actions that might lift the dust into the air of the work space and across the surface of an object. The goal is to remove the dust vertically without incidental imbedding or indention into the surface. Careful HEPA vacuum cleaning will collect dust from the object (see HEPA article). Aqueous and solvent treatments may increase rather than diminish the quantity of hazardous waste. Disposal should follow legally required procedures, as discussed in the recent article on Waste Management in AIC News [7].

Sampling and analysis of dust residues before and after treatment is strongly advised. General directions for sampling and for types of analytical request are outlined in Appendix 2 along with a short list of firms. Despite the desire to treat an object promptly, current information on the character of the WTC dust residues indicate that they are largely inert to fragile objects as long as this dust is not smeared, rubbed, or brushed in a manner that would allow it to act as an abrasive powder. The dust may contain disproportionate quantities of alkaline or acidic or carcinogenic material. Analysis will provide necessary data to develop the appropriate treatment for the individual object.

[1] ACTS Facts, vol. 15 #11 (November 2001):1-4.

[2] Brady, G.S. and H.R. Clauser, Materials Handbook, 12^th ed. New York: McGraw-Hill Book Company, 1987.

[3] Encyclopedia of Textiles, Fibers, and Nonwoven Fabrics, ed. M. Grayson. New York: John Wiley & Sons, 1984.

[4] Environmental Protection Agency www.epa.gov

[5] Kirk-Othmer Encyclopedia of Chemical Technology, 3^rd edition. New York: John Wiley & Sons, 1978.

[6] New York Committee for Occupational Safety and Health www.nycosh.org

[7] White, M., Bischoff, J.J., Stavroudis, C. and L. Goldberg,A>From Cradle to Grave,= Waste Management for Conservators@ AIC News Health & Safety Insert 3/1 in vol. 26 #6(November, 2001)

[8]Occupational Safety and Health Administration www.osha.gov

[9] Stoves, J.L. Fibre Microscopy: Its Technique and Application London: National Trade Press, 1957, pp. 258-9.

[10] Von Bergen W. and W. Krauss.Textile Fiber Atlas New York: American Wool Handbook Co., 1942, p. 35.

[11] Nazaroff, W.W., L. G. Salmon, and G.R. Cass, AConcentration and Fate of Airborne Particles in Museum,@ Environmental Sci. Technology, vol. 24 #1 (1990):66-77.

It is useful to review briefly the general nature of soil, and especially soil on woven textiles which attract dust. Standard soil cloths have been developed by manufacturers to determine the efficacy of cleaning equipment and detergents; these are based upon studies of dirt and soiling. Museum conservators and scientists have also studied the soiling environment, as have industrial engineers for cleaning equipment. An early major study of street soil showed variations of soiling among different American cities (Table I). The soils have fairly uniform inorganic components, perhaps due to a similarity of city building materials and street pavements; the non-combustible ash content is slightly over half the material. The most variable is the solvent soluble component--the oiliness of the soil--which was as low as 4.9% in Detroit and as high as 12.8% in St. Louis.

At the time this chart was developed, the dinginess caused by soiling could only be measured indirectly as a carbon black equivalence, compared visually to tinctorial pastes. However, a National Institute of Drycleaning study found that an 0.5 equivalence of carbon black as a pigment reduced the reflectance on white cotton by 81.1% and on white wool by 85.5%. Carbon black has excellent covering power.

Table I: Analyses of Normal Urban Dust from Various U.S. Cities (after Sanders & Lambert)

aAfter G.L. Kehl, The Principles of Metallographic Laboratory Practice. New York: McGraw-Hill, 1949, p. 233.