Energy conservation is a bottom-line issue for furnace manufacturers and users alike. With the cost of energy (in constant dollars) at a 30-year high, reducing energy consumption through the effective use of thermal insulation can produce a rapid payback. Furthermore, reduced energy consumption is usually synonymous with reduced carbon emissions.

Fig. 1. Forms of high-temperature thermal insulation

Thermal insulation comes in many forms, weights, strengths and materials. Figure 1 illustrates three forms of high-temperature thermal-insulation products – flexible, formable and rigid. The majority of these products contain fibers, granules or both. Additionally, some contain binders or cements to hold the basic constituents together, whereas others are mechanically bound (e.g., fused grains, interlocked fibers or quilted panels).

Until the early 1970s, natural asbestos was the most common reinforcing fiber in thermal insulation because of its superior strength, low weight and good thermal properties. Although a number of synthetic fibers have gained popularity in the intervening years, asbestos – with more than 3,000 uses at its peak – had broader commercial applicability than any fiber before or since. The most common high-temperature fibers in use today are fiberglass, alkaline earth silicate fiber, refractory ceramic fiber and carbon fiber.

A large body of literature exists on the human health consequences of asbestos exposure, and substantial research has also been performed on other insulation materials. This article focuses on five example products, each with a different fiber or grain type.

Industrial Hygiene Fundamentals

Over the first three-quarters of the 20th century, one of the best fibrous insulating materials was also the most hazardous – asbestos. Amphibole asbestos – common forms being brown (amosite) and blue (crocidolite) asbestos – is a known cause of mesothelioma, a rare lung cancer, in exposed workers. Serpentine asbestos – commonly called white (chrysotile) asbestos – has also been linked to cancer in exposed workers, primarily when found with amphibole fibers. Today, after more than eight decades of asbestos research and four decades of regulation, many products that contain other fibers undergo a high degree of scrutiny because of the public’s heightened awareness of the potential health hazards associated with asbestos.

When a design engineer is selecting an insulation material for an industrial heating application today, he is likely to consider whether or not the material poses a hazard to human health. A common practice is to review the manufacturer’s material safety data sheet (MSDS), which summarizes important information about toxicity and other hazards. The authors of this article strongly caution readers that reviewing MSDS and general technical articles such as this one is not a substitute for a worker safety program that is developed by a knowledgeable practitioner.

While MSDS usually provide substantial information about potential health hazards of a product, they typically are not a sufficient resource alone to evaluate health risks in a given application. In order to reduce risk, an industrial hygienist would utilize MSDS and technical references as well as their education and experience to develop an appropriate exposure-control and worker-protection plan. There are three steps in this process:
  • Anticipation and recognition
  • Evaluation
  • Control
Anticipation and Recognition
The first step in the industrial hygienist’s assessment program is “anticipation” and “recognition” of hazards. If fibers/particles are released during handling or use of the product, it is necessary to determine whether they are small enough to become airborne and enter the respiratory passages. The actual likelihood of inhalation is not quantified at this stage, but the possibility of inhalation is confirmed or rejected based on particle size (e.g., ball bearings cannot be inhaled but particles of talcum powder can). If particles are respirable, the next important consideration is whether or not they are retained or cleared from the lung, and if they are retained, how quickly tissue damage can occur.

During the “evaluation” step, the possible exposure pathways (inhalation, ingestion, transdermal) are evaluated for the anticipated use of the product. The most important steps are the estimation or measurement of exposure (e.g., quantity of material) from each pathway and the estimation of frequency of each type of exposure. Information on the toxicity of a material and the risks it may pose to workers is available from two sources: animal experiments and epidemiologic (worker tracking) studies. Also, reference must be made to relevant OSHA standards and other health-protection guidelines such as Threshold Limit Values (TLVs) published by the American Conference of Governmental Industrial Hygienists (ACGIH).

The last step is “control,” which can include product substitution and engineering controls (e.g., suction hoods adjacent to the point of particulate generation) and provide warnings to workers who may be handling the materials. If these controls are not feasible or effective, personal protective equipment (PPE) such as half-face respirators with particulate filter cartridges may still be necessary. Verification of engineering control effectiveness may require an additional evaluation step until verification has been completed, and PPE may still be warranted.

SIDEBAR: Glossary of Terms

Toxicity:Inherent property of an agent/chemical that makes it capable of producing an adverse effect

Hazard:Capacity of an exposure to an agent/chemical to produce a specific adverse effect

Risk:Probability or likelihood that an adverse outcome will occur when an individual or group is exposed to a particular concentration or dose of an agent/chemical

Possible Health Hazards

Underlying the industrial hygienist’s effort to protect worker health is the toxicologist’s understanding of the health hazards associated with different occupational exposures. Readers are again cautioned that the information below is not comprehensive, but rather illustrative of potential human-health concerns.

Refractory Ceramic Fibers (RCF)
NIOSH recently published a review of available RCF studies[1] on toxicology, epidemiology, occupational-medicine and industrial-hygiene information available on refractory ceramic fibers. In this report, NIOSH recommends that workplace exposures to RCF be maintained at less than 0.5 fibers per cubic centimeter of air (f/cc). Animal inhalation studies have showed that chronic, high exposure to RCF increased the incidence of mesothelioma in hamsters and lung cancer in rats. Epidemiologic studies of workers have not found an association between occupational exposure to airborne RCF and an excess rate of pulmonary fibrosis, lung cancer or mesothelioma. Experience with asbestos has shown, however, that long follow-up of worker populations is required to properly evaluate risks of lung cancer and mesothelioma. To date, RCF worker populations likely have not been followed for a sufficient length of time to properly evaluate cancer risks. Given these facts, prudent exposure-control procedures, including the use of PPE and other exposure controls, should be considered when handling products containing RCF.

Alkaline Earth Silicate (AES)
In 2001, the International Agency for Research on Cancer (IARC) concluded that certain AES fibers (which may be identified as mineral wool, slag wool or rock wool) should be downgraded from “possible human carcinogen” to “not classifiable as to carcinogenicity in humans.” The European criterion that distinguishes AES fibers from RCF is the alkali and alkaline earth oxide content in the material. If the sum of the oxides of sodium, potassium, magnesium, calcium and barium exceeds 18%, the material is more likely to dissolve in body fluids and be cleared from lung tissue, hence the name “biosoluble.”

Microporous Silica
“Microporous” (also known as “colloidal” or “fumed”) silica is an amorphous – non-crystalline – glassy material comprised of silicon dioxide. As discussed previously with respect to RCF, extended exposure of amorphous silica to high temperatures can cause the molecules to re-orient themselves and form cristobalite. However, the tiny particle size of the microspheres reportedly tends to inhibit grain growth.

Carbon Fiber Composite (CFC)
Graphite is not listed as a carcinogenic material by either IARC or OSHA, however, small graphite fibers or dust are suspected as being possible inhalation hazards. Graphite causes benign pneumoconiosis (graphitosis). Symptoms of pneumoconiosis (“dusty-lung-related” symptoms) from graphite exposure are dypsnea, coughing, black sputum, bronchitis and impairment of pulmonary function.

Vermiculite is a phyllosilicate mineral that is expanded (exfoliated) to improve its insulating properties. Vermiculite alone is not considered a highly toxic material, but in the past, some commercially sold vermiculite contained trace amounts of asbestiform materials. Exfoliated vermiculite may also contain minor amounts of crystalline silica, so prolonged high-level dust exposures should be controlled.


Table 1 presents engineering information from current data sheets of five commercially available thermal-insulation products.[2] These five materials do not exhaustively cover the hundreds of commercially available insulation products, but they do cover three of the main forms and several fiber or granule types.

Product A is an exceptionally high-temperature (2900°F) RCF blanket product manufactured from high-purity-alumina fibers. The product has low weight and low thermal conductivity. Compliance with guidelines for crystalline silica is recommended because the product may generate respirable dust and fibers.

Product B is a patented AES blanket product having a composition similar to slag or rock-wool products. It is designated as having “low biopersistence” due to its alkaline-earth silicate formulation. Exposures to this material could cause respiratory irritation and should be controlled to prevent airborne concentrations above 1 f/cc.

Product C is a filament-reinforced, microporous silica board product with exceptionally low thermal conductivity. Like any silica-containing materials that may encounter high-temperature conditions, the possibility of the formation of cristobalite should be considered. Exposures to this material should be controlled to prevent airborne concentrations above 1 f/cc.

Product D is a vacuum-formed, graphite-fiber product developed to withstand the temperatures and mechanical forces in rapid-quench vacuum furnaces. It has good strength and dimensional stability but higher thermal conductivity than the silicate-based materials. There has been limited research on the health effects of exposure to carbon fibers. It would be prudent to prevent exposures above concentrations of 1 f/cc.

Product E is a moderately high-temperature rigid block product, manufactured from vermiculite granules and binders. It has good compressive strength, high green strength and is 93% porous. This material could cause irritation if airborne concentrations are elevated. Some sources of vermiculite historically have contained trace concentrations of asbestos, so verification that the product is free of asbestos is appropriate.


While thermal insulation is a vital component of an energy-savings program, awareness of possible health hazards associated with the insulation materials is important for furnace builders and users alike. Five unique thermal-insulation products were described, and some information on potential health hazards was identified for these products. Fibers and granules can represent inhalation hazards if they become airborne and are small enough to be respirable. Respiratory cancers and pneumoconiosis are the most serious health consequences of overexposure to historically used insulation materials containing asbestos. The cancer risk for the five insulation materials described appears lower, however, based on available data. Some uncertainties remain with regard to the risks posed by RCF insulation, as workers making and using this material have not been followed for a sufficient time to reliably evaluate the risk of respiratory cancers with latency periods of 30 years and longer. IH

For more information:Contact Dr. Richard J. Martin, senior managing engineer for Exponent, Inc., 5401 McConnell Avenue, Los Angeles, CA 90066; tel: 310-754-2720; fax: 310-754-2799; e-mail:; web:

Mr. Del Malzahn, a board-certified industrial hygienist, is a senior managing scientist in Exponent’s Health Sciences Center for Public Health and Industrial Hygiene, Farmington Hills, Mich. Mr. Jeffrey Hicks, a board-certified industrial hygienist, is a principal scientist in Exponent’s Health Sciences Center for Public Health and Industrial Hygiene, Oakland, Calif. Dr. Patrick Sheehan is principal scientist in Exponent’s Health Sciences Center for Exposure and Dose Reconstruction, Oakland, Calif.

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at thermal insulation, refractory ceramic fiber, microporous silica, carbon fiber composite, vermiculite