Asbestos is a naturally-occurring fibrous material that can be woven into cloth, does not burn readily, has excellent properties for thermal insulation, and therefore came into common commercial use during this century.[2,pgs.390-392] Fiber glass has many of the same characteristics as asbestos.
In 1938, the Owens Corning Fiberglas Company was formed, and three years later, in 1941, evidence of pulmonary disease was reported by Walter J. Siebert, who investigated the health of workers with the cooperation of Owens Corning.[1,pg.292] That same year another investigator reported finding "no hazard to the lungs" of workers exposed to glass fibers in the air. Scientific disagreement of this sort has characterized the study of fiber glass ever since; meanwhile fiber glass production has increased steadily.
In 1941, the U.S. Patent Office issued patents for 353 glass wool products. Glass wool, fiber glass, fiberglas, fibrous glass, and glass fibers are all names for the same thing: man made thin, needle-shaped rods of glass.
Fiber glass is now used for thermal insulation of industrial buildings and homes, as acoustic insulation, for fireproofing, as a reinforcing material in plastics, cement, and textiles, in automotive components, in gaskets and seals, in filters for air and fluids, and for many other miscellaneous uses. More than 30,000 commercial products now contain fiber glass.
As asbestos has been phased out because of health concerns, fiber glass production in the U.S. has been rising. In 1975, U.S. production of fiber glass was 247.88 million kilograms (545.3 million pounds); by 1984 it had risen to 632.88 million kilograms (1392.3 million pounds).[1,pg.302] If that rate of growth (10.4% per year) held steady, then production of fiber glass in the U.S. in 1995 would be 436 million pounds.
Fiber glass is now causing serious health concerns among U.S. officials and health researchers. Dr. Mearl F. Stanton of the National Cancer Institute found that glass fibers less then 3 microns in diameter and greater than 20 microns in length are "potent carcinogens" in rats; and, he said in 1974, "it is unlikely that different mechanisms are operative in man." A micron is a millionth of a meter (and a meter is about three feet). Since that time, studies have continued to appear, showing that fibers of this size not only cause cancer in laboratory animals, but also cause changes in the activity and chemical composition of cells, leading to changes in the genetic structure in the cellular immune system. Although these cell changes may be more common (and possibly moreimportant) than cancer, it is the cancer-causing potential of glass fibers that has attracted most attention.
In 1970, Dr. Stanton announced that "it is certain that in the pleura of the rat, fibrous glass of small diameter is a potent carcinogen." The pleura is the outer casing of the lungs; cancer of the pleura in humans is called mesothelioma and it is caused by asbestos fibers. Stanton continued his research and showed that when glass fibers are manufactured as small as asbestos fibers, glass causes cancer in laboratory animals just as asbestos does.  Asbestos is a potent human carcinogen, which will have killed an estimated 300,000 American workers by the end of this century.  The finding that fiber glass causes diseases similar to asbestos was chilling news in the early 1970s, and an additional 25 years of research has not made the problem seem less serious. Workers in fiber glass manufacturing plants are exposed to concentrations of fibers far lower than the concentrations to which asbestos workers were exposed, yet several industry-sponsored epidemiological studies of fiber glass workers in the U.S., Canada, and Europe have reported statistically significant elevations in lung cancers. 
The International Agency for Research on Cancer (IARC), of the World Health Organization, listed fiber glass as a "probable [human] carcinogen" in 1987. In 1990, the members of the U.S. National Toxicology Program (NTP)-representatives of 10 federal health agencies-concluded unanimously that fiber glass "may reasonably be anticipated to be a carcinogen" in humans. NTP members were preparing to list fiber glass that way in the Seventh Annual (1992) Report on Carcinogens, the NTP's annual listing of cancer-causing substances, which is mandated by public law 95-622. But industry intervened politically.
Four major manufactures of fiber glass insulation campaigned for three years to prevent their product from being labeled a carcinogen by NTP. They managed to delay the publication of the NTP's Seventh Annual Report on Carcinogens for more than two years, but on June 24, 1994, the Secretary of Health and Human Services (HHS), Donna E. Shalala, signed the Report and sent it to Congress, thus making it official policy of the U.S. government that fiber glass is "reasonably anticipated to be a carcinogen." In the U.S., fiber glass must now be labeled a carcinogen.
Announcing this decision, government officials tried to play down its significance. Bill Grigg, a spokes-person for the U.S. Public Health Service (a subdivision of Health and Human Services) told the Washington Post ,"There are no human data I'm aware of that would indicate there's any problem that would involve any consumer or worker."  To make such a statement, Mr. Grigg had to ignore at least six epidemiological studies showing statistically-significant elevations in lung cancers among production workers in fiber glass factories. Indeed, according to researchers fiber glass is a more potent carcinogen than asbestos.[8,pg.580]
Fiber glass is now measurable everywhere in the air. The air in cities, rural areas,[1,pgs.311-314] and remote mountain tops  now contains measurable concentrations of fiber glass. If the dose-response curve is a straight line (that is to say, if half as much fiber glass causes half as much cancer) and if there is no threshold dose (no dose below which the cancer hazard disappears), then exposing the Earth's 5.7 billion human inhabitants to low concentrations of fiber glass will inevitably take its toll by causing excess cancers in some portion of the population.
According to OSHA researchers, an 8-hour exposure to 0.043 glass fibers per cubic centimeter of air is sufficient to cause lung cancer in one-in-every-thousand exposed workers during a 45-year working lifetime.[8,pg.580] In rural areas, the concentration of fiber glass in out-door air is reported to be 0.00004 fibers per cubic centimeter, about 1000 times below the amount thought to endanger one-in-every-thousand fiber glass workers.[1,pg.314] But people in rural areas breathe the air 24 hours a day, not 8 hours. Furthermore, a human lifetime is 70 years, not the 45 years assumed for a "work lifetime." Moreover, one-in-a-thousand is not adequate protection for the general public; U.S. Environmental Protection Agency uses one-in-100,000 or one-in-a-million as a standard for public exposures. (And in urban air, there's 10 to 40 times as much fiber glass as in rural air.) Therefore, the amount of fiber glass in the outdoor air in the U.S. and Europe (and presumably elsewhere) already seems higher than prudent public health policies would permit. Assuming a straight-line dose-response curve and no threshold, there is ample reason to be concerned about the human health hazards posed by fiber glass in the general environment.
It has been 25 years since researchers at the National Cancer Institute concluded that fiber glass is a potent carcinogen in experimental animals. During that time, additional research has confirmed those findings again and again. During the same period, the amount of fiber glass manufactured has increased rapidly year after year. Ninety percent of American homes now contain fiber glass insulation. All of this fiber glass will eventually be released into the environment unless special (and very expensive) precautions are taken to prevent its release. The likelihood of Americans taking such precautions is nil. Billions of pounds of fiber glass now in buildings will eventually be dumped into landfills, from which it will leak out slowly as time passes. Elevated concentrations of fiber glass are already measurable in the air above landfills today.
In 1991, Patty's Industrial Hygiene and Toxicology, a standard reference book on work-place safety and health, said about fiber glass, "...it is prudent for industrial hygienists to treat these materials with the same precautions as asbestos." [1,pg.324] How do we treat asbestos? In the U.S., all new uses of asbestos have been banned. A ban of fiber glass is long overdue.