The relationship between man-made mineral fibers such as fiberglass and glass wool and human health
1. Introduction
Asbestos is a widely used industrial material—a natural mineral fiber. However, asbestos poses a serious threat to human health and is listed in the United States as one of the ten most potent carcinogens among 86 industrial pollutants. Since the 1970s, when the global health and industrial communities recognized the health hazards of asbestos, it has been a major concern. Developed Western countries have enacted legislation to restrict or prohibit the processing, production, and use of asbestos and asbestos products. For example, in 1992, the U.S. Environmental Protection Agency announced a complete ban on the production of asbestos and asbestos products; today, no manufacturers in the U.S. produce asbestos. The World Trade Center in the United States used a large amount of asbestos for insulation and soundproofing, resulting in asbestos dust still drifting over the ruins after the 9/11 attacks. Experts are deeply concerned about the serious health consequences this asbestos dust poses to nearby American citizens. The European Union completely banned the use of asbestos in 2005, and currently, all Western European countries except Greece and Portugal have banned its use. my country and some developing countries still produce and use asbestos. However, the dangers of asbestos to human life and health are now a global consensus, and restricting or banning its use is a trend. Therefore, the development and application of man-made mineral fibers (MMMF) to replace asbestos has become a rapidly developing trend. With increasing environmental awareness and a growing awareness of self-health protection, people are not only concerned about banning asbestos products, but also about whether man-made mineral fibers, which have similar properties and forms to asbestos, pose a threat to human health. In China today, no one would believe the fabricated and absurd claim made during the Cultural Revolution in the revolutionary model opera "The Harbor" that ingesting glass fiber would "stick to the intestines and cause life-threatening danger." But what about the health risks of man-made mineral fibers such as glass fiber, glass wool, and rock wool? Will they cause cancer like asbestos? Will they cause silicosis like asbestos-induced asbestosis? Will they trigger other diseases? Extensive and in-depth research has been conducted worldwide, with consistent conclusions. However, research in my country is limited, and there is little reported technical literature on this topic, so it is normal for people to raise this question. This article explores this issue.
2. Man-made Mineral Fibers (MMMF), such as Glass Fiber, Glass Wool, and Rock Wool, and Asbestos
Man-made mineral fibers (MMMF) refer to fibrous industrial materials produced by artificially melting glass, rock, slag, or clay. They mainly include glass fiber (continuous glass fiber), glass wool (discontinuous glass fiber), rock wool, slag wool, and refractory fibers. Some refer to these fibers as man-made glass fibers (MMVF), but considering that some man-made mineral fibers are crystalline rather than glassy (such as crystalline refractory fibers produced using the colloidal method), this article uses the term man-made mineral fibers (MMMF).
Asbestos is a natural inorganic crystalline mineral fiber, and it is currently impossible to artificially synthesize asbestos fibers using scientific methods. Asbestos has a specific gravity of 2.4, is heat-resistant, fire-resistant, chemically resistant, acid and alkali-resistant, and moisture-resistant, and does not deteriorate or decay over time and with changing climates. Asbestos is a crystalline fiber, and when subjected to external force, it can break along its axis into even finer fibers. Asbestos fibers typically have a diameter of only 0.02–2 μm. Asbestos has been widely used in friction materials, electrical, acoustic, and thermal insulation, and in building applications, with asbestos cement products being a representative example.
Made-in-the-molecular-weight mineral fibers (MMMF) are cylindrical fibrous materials produced by melting mineral materials at high temperatures of 1000–1800℃. They are mostly amorphous, with atoms not arranged in a regular lattice like in crystalline materials. Fiber diameter varies greatly depending on the manufacturing process. For example, continuous glass fibers have a diameter of 3–25 μm, mainly 6–15 μm (6–13 μm in my country); glass wool fibers have a diameter of 3–10 μm; rock wool and slag wool fibers have a diameter of 2–6 μm; and refractory fibers have a diameter of 1.2–3.5 μm. Except for continuous glass fibers, most man-made mineral fibers (MMMF) are short fibers, with their length varying depending on their application and manufacturing method. For example, glass wool fibers are mostly 30-50 mm long, much longer than asbestos fibers. These man-made mineral fibers are not crystalline and cannot break into finer fibers along the axial direction. However, most of these fibers are brittle (especially coarse fibers) and easily break into shorter fibers under stress. Man-made mineral fibers generally have a specific gravity of around 2.5-3, high strength, high elastic modulus, are non-flammable, non-moldy, acid and alkali resistant, and have good electrical, acoustic, and thermal insulation properties, finding wide application in reinforcement, thermal insulation, and other fields.
In summary, asbestos and man-made mineral fibers (except continuous fibers) are both short fibers, with similar appearance, properties, and applications. The difference lies in their composition and structure. Asbestos fibers are crystalline, while the latter have an amorphous, non-crystalline structure. The ability of asbestos fibers to break into finer fibers along the axial direction is crucial to human health.
3. Conditions for Human Inhalation of Fibers
People are not only concerned about which fibers are more easily inhaled, but also about the potential toxic and pathological changes that inhaled fibers may cause in the body.
Many factors influence the inhalation of fibers, such as the physical morphology and geometric dimensions of the fibers, and their concentration in the air. The toxic and pathological changes produced by fibers after entering the human body are limited by the fiber's resistance to corrosion by bodily fluids. These three aspects will be discussed below.
3.1 Physical Morphology and Geometric Dimensions of Fibers
During the mining, crushing, removal, sorting, and production of asbestos products and their applications, large quantities of asbestos fibers enter the air. Similarly, fibers from man-made mineral fibers such as glass fiber, glass wool, rock wool, and slag wool also enter the air during production and application, and thus, all of these can potentially be inhaled. However, due to differences in fiber diameter, surface condition, and length, the probability of inhalation varies.
Asbestos is a natural mineral material—a crystalline fibrous material. Under external force, it breaks along its length into finer fibers. This means that the diameter of asbestos fibers can continuously change during processing and use, reaching extremely fine dimensions, such as below 0.02 μm. Continuous glass fibers typically have a diameter of 6–13 μm and a continuous length; glass wool fibers typically have a diameter of 3–10 μm; rock wool and slag wool fibers typically have a diameter of 2–6 μm; and aluminosilicate refractory fibers typically have a diameter of 1.2–3.5 μm. These fibers break under external force during processing and application, reducing their length while their diameter remains unchanged.
When a person breathes air, some longer fibers floating in the air are filtered by the nasal hairs and mucous membranes of the nose, bronchi, and trachea. This means that not all fibers floating in the air are inhaled. According to long-term experimental research by the World Health Organization (WHO), the U.S. Institute for Occupational Safety and Health, the Insulation Materials Manufacturers Association, and many experts worldwide, the minimum diameter of fibers inhaled by the human body should be less than 3 μm, with a length-to-diameter ratio greater than 5:1. It is generally believed that fibers longer than 200–250 μm will not be inhaled deep into the lungs. Some experts believe that fibers longer than 100 μm are almost impossible to inhale deep into the lungs.
In other words, commonly used continuous glass fibers (diameter 6–13 μm) are unlikely to be inhaled deep into the lungs; for glass wool, the diameter is 3–10 μm, and for rock wool and slag wool, the diameter is 2–6 μm, and their length is much greater than 200 μm. Therefore, from the perspective of fiber physical morphology and geometric dimensions, it is very difficult for man-made mineral fibers such as glass fibers, glass wool, rock wool, and slag wool to be inhaled deep into the lungs.
3.2 Fiber Concentration in Air
The higher the fiber concentration in the air, the higher the probability of fibers being inhaled into the human body.
Due to the inherent structural characteristics of asbestos fibers, during production and application, they are prone to breakage under external forces, not only along their length into finer fibers but also perpendicular to the axial direction into even shorter fibers. Combined with the nature of asbestos mining, production, and processing, the concentration of asbestos fibers in the air is very high, typically tens, hundreds, or even thousands of fibers per cubic centimeter of air. Because these fibers are fine and short, they float in the air for a long time, making it obvious that people are more likely to inhale them in such environments.
During the continuous glass fiber drawing process, a sizing agent is applied to the surface of the new fibers before they are wound onto a bobbin and sent to subsequent processing steps. The sizing agent consists of lubricating components, film-forming components, antistatic components, and other auxiliary materials. As an aqueous solution, the sizing agent protects the new fibers during the drawing process, thus generally preventing short fibers from entering the air. For textile glass fibers, during processes such as unwinding, twisting, warping, and weaving, the fibers are subjected to external forces. Because the lubricating components in the sizing agent provide lubrication and protection, the amount of fiber breaking into short fibers and escaping into the air is very limited. For glass fiber reinforced materials, after the filament bobbins are dried, the sizing agent forms a uniform protective film with a certain strength and toughness on the fiber surface. Therefore, fiber breakage and fuzzing are minimal during processing and use. In these environments, the fiber concentration is very low, generally less than one fiber per cubic centimeter of air volume. Furthermore, due to the fiber diameter range of 6–13 μm, the fibers are relatively coarse and have a fast settling velocity in the air, resulting in a short residence time.
For the production of man-made mineral fibers such as glass wool, rock wool, and slag wool, the application of binders at the fiber-forming points significantly reduces fiber dust (e.g., glass wool products typically contain 5%–10% binder). Fiber dust is more prevalent at cutting and packaging areas; adding protective measures in these areas can further reduce fiber dust emissions. Similarly, these fibers are mostly larger than 3μm in diameter and relatively long, resulting in faster settling speeds in the air. Therefore, the fiber concentration in the ambient air is much lower than that in the environments where asbestos is produced, processed, and used.
Research and statistical analysis show that the fiber concentration in the air during the production and application of MMMF fibers (such as glass fiber, glass wool, rock wool, and slag wool) is low, generally maintained at around 1 fiber/cm³, hundreds of times lower than that of asbestos. Due to the low concentration of fibers in the air, the probability of MMMF fibers being inhaled into the lungs is very small.
3.3 Fiber's Resistance to Lung Fluid Erosion
Having discussed the probability of fibers being inhaled, the most pressing question is how long fibers can remain in the lungs once inhaled. Ideally, the shorter the retention time, the better. The duration of fiber's presence deep in the lungs depends on its chemical stability, i.e., its resistance to lung fluid erosion, which plays a crucial role in its potential biological effects.
The chemical stability of fibers, i.e., their dissolution rate in chemical solutions, mainly depends on their chemical composition, surface area, and surface state.
Asbestos fibers are crystalline silicate materials. For example, chrysotile asbestos, with the molecular formula Mg₃(Si₂O₅)(OH)₄, has a plate-like crystal structure composed of alternating layers of silicon-oxygen tetrahedral plates and magnesium hydroxide octahedral plates, forming coiled tubular fibers connected by hydroxyl (OH) ions and magnesium ions. Asbestos exhibits good chemical stability and resistance to acids and alkalis, especially alkalis. Human lung fluid has a pH of 7.5, which is slightly alkaline, and asbestos fibers show strong resistance to lung fluid corrosion. Studies have shown that the asbestos dissolution rate constant is 0.09 ± 0.005 ng/cm²·h. Asbestos is a highly surface-active mineral fiber. The ends of chrysotile asbestos fibers contain unsaturated O-Si-O, Si-O-Si, and Mg-O bonds, with the exposed O²- groups exhibiting particularly high reactivity. Furthermore, the columnar surfaces of asbestos fibers contain highly reactive (OH) groups and some dangling bonds. Therefore, asbestos fibers are highly reactive, which is why they can cause cancer in humans.
Glass fibers are mainly classified into several categories based on their composition, including E, C, A, and S. These are all amorphous silicate materials containing various metal oxides and non-metal oxides, with SiO₂ as the main component. Studies of these oxides have shown that they significantly reduce the dissolution rate. B₂O₃, BaO, Na₂O, CaO, and MgO increase the dissolution rate, with B₂O₃ having the greatest effect. SiO₂ has little effect on the dissolution rate.
During the drawing and blowing processes of glass fibers and glass wool, the nascent fibers develop microcracks on the surface of the glass fiber cylinders due to thermal stress. Furthermore, the glass fiber surface contains some cationic substances and is hydrophilic, making it easily wetted by lung fluid. Lung fluid has a pH of 7.5 and is slightly alkaline. These microcracks on the glass fiber surface expand and deepen under the corrosive effect of lung fluid, increasing the surface area of the glass fiber and decreasing its strength, thus accelerating its dissolution by macrophages.
Studies have calculated that the glass fiber dissolution rate constant is 50–300 ng/cm²·h.
4. Man-made mineral fibers such as glass fiber, glass wool, rock wool, and slag wool pose no threat to human life.
4.1 Theoretically, man-made mineral fibers such as glass fiber do not endanger human life.
The conditions for inhaling fibers discussed above are: fiber diameter <3μm, aspect ratio greater than 5:1, and length less than 100μm. Glass fiber does not meet these conditions and is almost impossible to inhale deep into the lungs. Glass wool, rock wool, and slag wool also contain very little fiber dust that meets these conditions, making the probability of them being inhaled deep into the lungs extremely small. Even if this fiber dust is inhaled, it will dissolve within the body within a few months, at most 2-5 years. Even for industrial workers, the concentration of fiber dust in the ambient air is low, resulting in even less fiber that can be inhaled. The rate (quantity) of fiber inhalation is lower than the rate of dissolution. Therefore, theoretically, these man-made mineral fibers do not pose a threat to human life.
4.2 Conclusions from Animal Experiments
Since the discovery that asbestos is one of the ten most carcinogenic industrial materials harmful to human health, scientists worldwide have conducted extensive animal experiments to study the potential hazards of man-made mineral fibers such as glass fiber to human health.
① Lung poisoning experiments on animals. Hamsters were exposed to high concentrations of glass wool dust. For example, Lebouffant exposed mice to 5 mg/m³ of inhalable fiber for 12–24 months in 1987; Wagner exposed mice to 10 mg/m³ of inhalable fiber for 50 weeks in 1980 and 1984; Bunr et al. exposed mice to 30 mg/m³ of fiber in 1993, and subsequent autopsies showed no fibrotic lesions in the mice.
② Lung carcinogenicity experiments on animals. Rats and hamsters were exposed to high concentrations of glass wool dust for extended periods.