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reducing tobacco cigarette consumption, per- haps more than other measures, including mandatory health warnings, advertising bans on radio and television, and innumerable other efforts instituted by public health and medical professional organizations. But, has the ETS social movement been based on scientific truth and on reproducible data and sound scientific principles? At times, not surprisingly, the ETS social movement and scientific objectivity have been in conflict. To start with, much of the research on ETS has been shoddy and poorly conceived. Editorial boards of scientific journals have selectively accepted or excluded contributions not always on the basis of inherent scientific merit but, in part, because of these social pres- sures and that, in turn, has affected and biased the data that are available for further analyses by professional organizations and governmen- tal agencies. In addition, "negative" studies, even if valid, usually are not published, espe- cially if they involve tobacco smoke, and thus they do not become part of the whole body of literature ultimately available for analysis. Negative results on ETS and health can be found in the scientific literature, but only with great difficulty in that they are mentioned in passing as a secondary variable in a "positive" study reporting some other finding unrelated to ETS. To evaluate critically any potential adverse Figure 1: Particulate Phase and Gas Phase of Tobacco Smoke* health effects of ETS, it must first be appreciat- ed that not all tobacco smoke is the same, and thus the risk for exposure to the different kinds of tobacco smoke must be considered indepen- dently.l What Is ETS? The three most important forms of tobacco smoke are depicted in Figure 1. Mainstream smoke is the tobacco smoke that is drawn through the butt end of a cigarette during active smoking; this is the tobacco smoke that the active smoker inhales into his or her lungs. The distribution of mainstream smoke is sum- marized in Table 1 (page 12). Sidestream smoke is the tobacco smoke that is released in the sur- rounding environment of the burning cigarette from its smoldering tip between active puffs. Many publications have treated sidestream smoke and ETS as if they were one and the same, but sidestream smoke and ETS are clear- ly not the same thing. Sidestream smoke and ETS have different physical properties and they 1A burning cigarette has been described as "a miniature chemical factory," producing numerous new components from its raw materials. When a cigarette is smoked, the burning cone has a temperature of about 860 to 900°C during active puffing, and smolders at 500 to 600°C between puffs. When tobacco burns at these temperatures, the products of pyrolyzation are all vapors. As the vapors cool in passage away from the burning cone, they condense into minute liquid droplets, initially about two ten-millionths of a meter In size. Generally, then, all forms of smoke are microaerosols of very small liquid droplets of particulate matter suspended in their surrounding vapors or gases. Thus, all smoke has a "particulate phase" and a "gas phase." 0 000000000000000000000 000000000000000000000 00000000000000000 C o C o 0oooooaooooo000000000 000000000000000000000 000000000000000000000 000000000000000000000 0 0 000 0 0 0 0000 0000 G 00 000000 00 0000000 000 000000000 00 0 0 0 0 0 Mainstream Smoke C C 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 Sidestream Smoke Environmental Tobacco Smoke (ETS) * Schematic representation of the particulate phase and the gas phase of tobacco smoke. Environmental tobacco smoke is not smoke In the conventional sense, but rather a very limited number of highly-diluted remnants or residual constituents of mainstream smoke and sidestream smoke. July 1991 11

.Tab1e 1: Distribution of Mainstream Smoke Total Mainstream Smoke 500* Wet Totai Particulate Matter 22 Nicotine 1.3 Water 3.7 "Tar" , 17 Aerosol Gas Phase Water 478 Air Components 50 Carbon Monoxide 350 Carbon Dioxide 50 Other Components 8 'Alf data expressed In milligrams for a 501 mg deliver cigarette, as deter- mined by Federal Trade Commission criteria. SOURCE: Adapted from Huber, 1989. have different chemical properties. Environ- mental tobacco smoke is usually defined as a combination of highly diluted sidestream smoke plus a smaller amount of that residual main- stream smoke that is exhaled and not retained by the active smoker. What really is ETS? In comparison to mainstream smoke and side- stream smoke, ETS is so highly diluted that it is not even appropriate to call it smoke, in the conventional sense. Indeed, the term "environ- mental tobacco smoke" is a misnomer. Why is ETS a misnomer? Several reports on smoking and health from the Surgeon General's Office, a National Research Council review of ETS in 1986, the more recent Environmental Protection Agency's risk assess- ment of ETS, and several review articles all have provided a long list of chemical con- stituents derived from analyses of mainstream smoke and sidestream smoke, with the implica- tion that because they are demonstrable in mainstream smoke and sidestream smoke these same constituents must, by inference, also be present in ETS. No one really knows if they are present or not. In fact, most are not so present or, if they are, they are present only in very dilute concentrations that are well below the level of detection by conventional technologies available today. Only 14 of the 50 biologically active "proba- ble constituents" of ETS listed by the Surgeon General, for instance, actually have been mea- sured or demonstrated at any level in ETS. The others are there essentially by inference, not by actual detection or measurement. Thus, there are 36 constituents in these lists that are in- ferred to be present in ETS, but their presence has not been confirmed by actual detection or measurement. In this sense, then, ETS is really not smoke in the conventional sense of its defi- nition, but rather consists of only a limited number of "remnants" or residual constituents present in highly dilute concentrations. Because the levels of ETS cannot be quanti- fied accurately as such in the environment, some investigators have attempted to measure one or more constituent parts of ETS as a "sub- stitute marker" for ETS as a whole. The most frequently employed such "marker" has been nicotine or its first metabolically stable break- down product, cotinine. Nicotine was consid- ered an "ideal marker" because it is more or less unique to tobacco, although small amounts can be found in some tomatoes and in other food sources. In the mainstream tobacco smoke that is inhaled by the active smoker, nicotine starts out almost exclusively in the tiny liquid droplets of the particulate phase of the smoke. Because the smoke particles of ETS become so quickly and so highly diluted, however, nicotine very rapidly vaporizes from the liquid suspend- ed particulates and enters the surrounding gas. In technical terms, the process by which nico- tine leaves the suspended aerosol particle to enter the surrounding gas phase is called "denudation." As a vapor or gas, nicotine reacts with or adsorbs onto almost everything in the environ- ment with which it comes into contact. Thus, nicotine is not a representative or even a good surrogate marker for the particulate phase, or even the gas-vapor phase, of ETS. In fact, there are no reliable or established markers for ETS. The remnant or residual constituents of ETS each have their own chemical and physical behavior characteristics in the environment and none is present in a concentration in our environment that reaches an established threshold for toxicity.2 Measuring Health Risks Because the level of exposure to ETS or the dose of ETS retained cannot be quantified under every-day, real-life conditions, the health effects following exposure to residual con- 2A threshold limit value (usually expressed as milligrams of a substance per cubic meter of air or as parts of a substance present per million parts of res- pirable clean air) is the recommended concentration of a substance as the maximal level that should not be exceeded to prevent occupational disease through exposure in the workplace. Threshold limit values have not been established for our general, every-day environment outside of industrial expo- sure. Threshold limit values are determined by toxicologists, epidemiologists, and hygienists through their interpretation of literature, and usually are sanc- tioned by the American Conference of Governmental Industrial Hygienists. No constituent of ETS has been measured in our every-day environment at levels that exceed the threshold limit values permitted in the workplace. 12 Consumers' Research

stituents of ETS have been impossible to evalu- ate directly. In broad terms, two different approaches have been employed in an attempt to assess indirectly the health risks for expo- sure of the nonsmoker to the environmental remnants of ETS. The first of these involves a theoretical concept that is called "linear risk extrapolation." Linear risk extrapolation has been employed extensively in attempts to deter- mine the risk for lung cancer in nonsmokers exposed to ETS.3 This concept of linear risk assumes that if there is a definable health risk for the active smoker, then there also must be a projected lower health risk for the nonsmoker exposed to ETS. This is represented schematically in Figure 2. The risk has been presumed to be lin- ear from the active smoker to the nonsmoker exposed to ETS, based proportionately on the relative exposure levels and retained doses of smoke; it thus requires some measurement of tobacco smoke exposure for both groups. This is fairly easy to achieve in the active smoker, in part because mainstream smoke has been so well-characterized and it is delivered directly from the butt-end of the cigarette into the smoker. Such is obviously not the case, howev- er for the nonsmoker exposed to ETS. Most projections of linear risk for ETS-expo- sure have been based on the use of nicotine as a representative marker of exposure. A few pro- jections have been based on carbon monoxide levels or amounts of respirable suspended par- ticulates in the environment, but these approaches are fraught with even greater error. Since nicotine initially is in the particulate phase of the mainstream smoke inhaled by the active smoker and it is present primarily as a highly diluted gas-phase remnant or residual vapor-phase con- stituent in the nonsmoker's environment, the concept of a linear health risk from the active smoker to the nonsmok- er is based on rather shaky s.;ientific-reasoning. That is to say, it is not valid to estimate a health risk for exposure to the particulate phase in the active smoker and then compare it with the health risk for exposures to the gas phase in the ETS- exposed nonsmoker. Simply stated, "like" is not being com- Figure 2: Linear Risk Extrapolation* 5.0 ~ 03.0 No Threshold One Molecule Theory pared to "like." Mainstream smoke and the residual constituents of ETS represent very dif- ferent exposure conditions. Whether present in mainstream smoke or in ETS, particulate phase and gas phase constituents have very different biological properties, as well as different physi- cal and chemical characteristics, and any asso- ciated health risks are also very different. The concept of linear risk extrapolation for ETS is based on a theory that when applied to ETS incorporates unsound assumptions that are not valid. There is no way, as yet, to evaluate or compare the levels of exposure in active smok- ers and nonsmokers exposed to ETS. The second approach used to evaluate health risks for nonsmokers exposed to ETS has employed epidemiologic studies. Epidemiology is a branch of medical science that studies the distribution of disease in human populations and the factors determining that distribution, chiefly by the use of statistics. The chief func- 3The concept is based on a theoretical extrapolation of the risk for lung cancer in the active smoker to the risk for lung cancer in the passive smoker on the basis of a "representative marker" for both smoke exposures. This "linear risk extrapolation" from one to the other is a model that is based on mathematical theory and on several assumptions. The theory assumes that the risk applies to all exposure levels, even if they are very low. Some advocates of the model even assume a "one molecule, one hit" mechanism, where exposures so low that they cannot be detected or measured can still cause disease if only a sin- gle molecule reaches a vulnerable body tissue. The linear risk theory also assumes that the risk for accumulative exposure remains constant and, thus, that the exposed individual has no capacity to adapt or develop tolerance mechanisms for the exposure. Since active smokers readily and rapidly devel- op tolerance through a variety of defense mechanisms, it seems illogical to assume those repeatedly exposed to ETS would not do the same. The linear risk model assumes that the risk for exposure to ETS is independent of any confounding factors. Finally, for this theory to be valid, it must be assumed that the risk Is linear for duration of exposure and that it is linear for concen- tration of exposure. None of these assumptions holds true on scientific testing for comparative projections of mainstream smoke to ETS. 1.0 0.0 0 2.0 4.0 6.0 8.0 10 Relative Environmental Exposure Level 'The concept of linear risk extrapolation. In this theory, the health response (expressed as a rela- tive risk) is directly or linearly related to the relative environmental exposure level. This theory sug- gests that there Is no "safe" threshold below which there is no response, and that exposure to as little as one molecule of the environmental substance can cause an adverse response. July 1991 13

I "Of the 30 ETS-lung cancer stud- ies, 6 reported a statistically significant association... and 24 of those studies reported no statistically significant effect." tion of epidemiology is the identification of pop- ulations at high risk for a given disease, so that the cause may be identified and preventative measures implemented. Epidemiologic studies are most effective when they can assess a well-defined risk. Because ETS-exposure levels cannot be mea- sured or in any other way quantified directly, even by representative markers, epidemiolo- gists have had to use indirect estimates, or sur- rogates, of ETS exposure. For nonsmoking adults, the number of active smokers that are present in the household has been used as a surrogate for ETS exposure. Usually the active smoking household member has been the non- smoker's spouse. With a few limited exceptions, disease rates in nonsmokers exposed to a spouse who smokes have been the basis for all epidemiologic assessments. Almost all of these studies have evaluated nonsmoking females married to a husband who smokes. For children, the surrogate for ETS exposure has been the number of parents in the household who smoke. Estimates of ETS expo- sure based on spousal or parental surrogates have been derived by various questionnaires; no study employs any direct quantification of ETS or of ETS remnant constituents in the actual environment of the nonsmoker. Questionnaires of smoking habits are notori- ously limited and often inaccurate, in part because of the "social taboo" that smoking has become and, in part, for other reasons related to the ETS social movement. Nevertheless, data from questionnaires about smoking behavior in spouses or in parents are the only estimates of ETS exposure available. Rates for three dis- eases in nonsmokers exposed (via surrogates) to ETS have been assessed: lung cancer, coro- nary heart disease, and respiratory illness in infants and small children. Only lung cancer will be discussed in this article. ETS and Lung Cancer What is the state of evidence on ETS and lung cancer? Almost all of the epidemiologic studies that are available to answer that ques- tion are based on the concept of some measure- ment of relative risk. None of the studies actu- ally has measured exposure to ETS or to any of its residual constituents directly. Relative risk is a relationship of the rate of the development of a disease (such as lung cancer) within a group of individuals exposed to some variable in the population studied (such as ETS) divided by the rate of the same disease in those not exposed to this variable. Relative risk is most frequently expressed as a "risk ratio," which is a calculated comparison of the rate of the disease studied in the exposed population divided by the rate of that disease in some control population not exposed to the variable studied. The terms "risk ratio" and "relative risk" are often used synonymously. Thus, the relative risk in all epidemiologic ETS studies on lung cancer is expressed as the rate of lung cancer in the ETS-exposed group (indi- viduals married to a household smoker) divided by the rate of lung cancer where there was no ETS exposure (no household smokers). If the disease rates were exactly the same in these two groups, the risk ratio would be 1.0. There have been 30 epidemiologic studies on spousal smoking and lung cancer published in the scientific literature. Twenty-seven of these epidemiological studies were case control stud- ies, where the effect of exposure to spousal smoking was evaluated retrospectively on data that had already been available for review. The "cases" in these case-control studies were non- smoking individuals with lung cancer married to smokers. The rate of lung cancer in these "cases" was compared, by the derived risk ratio, to the rate of lung cancer in "control" or nonsmoking individuals who were married to nonsmokers. Three of the studies followed cohort popula- tions of individuals exposed to spousal smoking prospectively over the course of time. A "cohort" is any designated group of people. A "cohort study" identifies a group of people that will be exposed to a risk and a group that will not be exposed to that risk, and then follows these groups over time to compare the rate of disease development as a function of exposure or no exposure. The first studies were published in 1982 and the last studies were published in 1990. The studies originate broadly from different parts of the world and, for the most part, involve evalu- ations of lung cancer in nonsmoking females married to a smoking male partner; eight of the studies have limited data on nonsmoking males married to smoking females. Some of the stud- 14 Consumers' Research

ies are quite small, listing fewer than 20 sub- jects; otliers are based on larger populations, with four studies reporting between 129 and 189 cancer cases. Of the 30 studies, six reported a statistically significant association (identified by a positive relative risk ratio in the spousally- exposed to the non-exposed population) and 24 of the studies reported no statistically signifi- cant effect. The average esti- mated relative risk ratio for each study and each sex is list- ed in Table 2, as are the confi- dence intervals reported by the authors or, where not reported, calculated by others in pub- lished review articles.4 Some of the negative studies- that is, some of the 24 studies that did not show a statistically significant association between the development of lung cancer and exposure to spousal smok- ing-contained data that sug- gested to the authors or to other reviewers a "positive trend." In most of science, "trends" do not count; data stand as either sta- tistically significant or not sta- tistically significant, with sig- nificance determined by specif- ic accepted rules of biostatis- tics. New rules should not be "made to fit" an otherwise unproved hypotheses, just because the subject is tobacco and the observed results do not support the hypothesis investi- gated. ETS Risk Weak A relative risk is called strong or it is called weak, depending on the degree of association, or the magnitude of the risk ratio. A strong relative risk would be reflected by a risk ratio of 5 to 20 or greater. Weak relative risks, by conventional defini- tion, have risk ratios in the range of 1 to 3 or so. Within 4A confidence interval is a range of values that has a specified probability of including the true value (as opposed to the estimated average value) within that range. In the data presented in Table 2, the confidence intervals are set such that there is a 95% probability that the true value will fall within the range of values listed. the 30 epidemiologic studies on ETS and lung cancer, there are 37 different total reported sets of risk ratios for male or female nonsmok- ers. None of the studies reports a strong rela- tive risk. Nine of the studies report risk ratios of less than 1.0. Thus, the results from all epidemio- (See SMOKE, page 33.) Table 2: Studies of ETS and Lung Cancer in Nonsmokers Study 95% Number Relative Confidence Sex of Cases Risk* Interval Case Control Studies Chan and Fung, 1982 F 34 0.75 (0.43, 1.30) Trichopoulos et a1.,1983 F 38 2.13** (1.18, 3.83) Correa et a1.,1983 F 14 2.07 (0.81, 5.26) M 2 1.97 (0.38, 10.29) Kabat and Wynder, 1984 F 13 0.79 (0.25, 2.45) M 5 1.00 (0.20, 5.07) Buffter et a1.,1984 F 33 0.80 (0.34, 1.81) M 5 0.51 (0.15, 1.74) Garfinkel et al., 1985 F 92 1.12 (0.94, 1.60) Wu et al., 1985 F 29 1.20 (0.50, 3.30) Akiba et ai., 1986 F 73 1.52 (1.00, 2.5) M 3 2.10 (0.5, 5.6) Lee et ai.,1986 F 22 1.03 (0.37, 2.71) M 8 1.31 (0.38, 4.59) Brownson et a1.,1987 F 19 1.68 (0.39, 2.97) Gao et a1.,1987 F 189 1.19 (0.6, 1.4) Humbie et a1.,1987 F 14 1.78 (0.6, 5.4) Koo et a1.,1987 F 51 1.55 (0.87, 3.09) Lam et ai., 1987 F 115 1.65** (1.16, 2.35) Pershagen et a1.,1987 F 33 1.20 (0.70, 2.10) Geng et a1.,1988 F 34 2.16 ** (1.03, 4.53) Inoue and Hirayama, 1988 F 18 2.55 (0.91, 7.10) Katada et al., 1988 F 17 - (NS;p=0.23) Lam and Cheng, 1988 F 37 2.01** (1.12, 1.83) Shimizu et a1.,1988 F 90 1.10 N/A He, 1990 F 45 0.74 (0.32, 1.68) Janerich et a1.,1990 F 129 0.93 (0.55, 1.57) Kabat, 1990 M 13 1.20 (0.54, 2.68) F 35 0.90 (0.46, 1.76) Kalandidi et a1.,1990 F 91 2.11 (1.09, 4.08) Sobue et a1.,1990 F 64 0.94 (0.62, 1.40) Svensson, 1990 F 17 1.20 (0.40, 2.90) Wu-Williams et al., 1990 F 205 0.7 (0.6, 0.9) Cohort Studies Garfinkei, 1981 F 88 1.17 (0.85, 1.89) (0.77, 1.61) Gillis et a1.,1984 F 6 1.00 (0.59, 17.85) M 4 3.25 Hirayama, 1984b F 163 1.45 (1.04 2.02) 1984a 7 2.28** (1.19 4.22) *Weak relative risks have risk ratios of between 1 and 3, or so. Any risk ratio below I represents a nega- tive relationship. Note that none of the studies show a strong relative risk. ** Statistically significant at the 5% level. July 1991 15

What to Know When Shopping for Tires oavidavs,rak he purchase of tires, depending on your vehicle and needs, can run several hun- dred dollars or more, and affects your everyday driving safety. So the more you know about the subject, the better the chances are that you will get exactly what you need-noth- ing more and nothing less. In the long run, this will save you time and, more importantly, money-while still keeping your vehicle com- fortable and safe to drive. The following article addresses some impor- tant considerations in tire purchase, including tire sizing, tire type, and warranty offerings. Tire Sizing Much of what you need to know about tires is printed right on the sidewall. This information includes the tire's size, as well as safety notices about load-carrying capacity, maximum infla- tion pressure, the Department of Transpor- tation certification, and descriptions of the materials used in the make-up of the tire. Sizing is one of the most misunderstood and understandably confusing parts of buying tires. The most widely used sizing system is the "P- Metric" system. To understand how this system works, let's choose one of the most common tire sizes today. Under the "P-Metric" system, this tire size is listed as "P195/75R14" on the sidewall. The P stands for passenger and means that the tire was designed and rated for use on a passenger vehicle. A light-truck tire would be represented by "LT." The 195 represents the "section width" of the tire, which is the sidewall to sidewall width, expressed in millimeters and measured at the widest point of the sidewall, when the tire is mounted on the correct size wheel. The 75 represents the aspect ratio of the tire, which is the ratio between the sidewall height and the section width of the tire. In this exam- ple, the sidewall is 75% as tall as the tire is Mr. Bystrak, who is certified by the National Institute of Automotive Service Excellence, is a tire dealer in Buffalo, New York. This article is adapted from his booklet "The Smart Shopper's Guide to Buying Tires," available for $5.95 from Info Industries, P.O. Box 1005, Buffalo, N.Y. 14224. (Bulk orders available.) wide, or 75% of 195 millimeters. The smaller the aspect ratio-also known as series or pro- file-of the tire, given the same section width, the shorter the tire. For example, a "60 series" tire would be shorter than a 70 series. Also, as the height of the tire decreases with the small- er series, the actual tread width will increase. (When a tire has no series designation-for example the European sized 165R13-the aspect ratio is understood to be 82.) The R indicates that the tire is a "radial." Other tire types are indicated by D for diagonal ply, or B for bias belted. These tire types are discussed below. The 14 is a measure of the rim diameter. Most common passenger tires come in 13-, 14-, or 15-inch rims, though some newer, higher performance cars are coming with 16- and 17- inch rims. Some high-performance tires carry an alpha- betic symbol denoting a speed rating. These let- ters correspond to a sustained performance level that the tire is capable of achieving safely. Speed-rated tires resist heat better than non- speed-rated tires, generally because of greater internal strength and reinforcement, but also tend to give a harsher ride. The most common speed ratings are as follows: S, rated to speeds up to 112 miles per hour (mph); T, rated to 118 mph; H, rated to 130 mph; V, rated to 149 mph; and Z, rated to 149+ mph. Changing Sizes W~kb While in most cases you may simply want to replace the tires the manufacturer provided with your car, you need not rule out changing ~ to a new tire size. But care must be taken when changing tire ~ sizes or aspect ratios so as not to undersize the ~, 16 Consumers' Research

tire for the car. Passenger tires of the exact same size can carry the same amount of load, no matter the manufacturer or design. A larger size tire, all other things being equal, will have a greater load-carrying capacity than a smaller size tire, because it holds more air. Load carry- ing capacity is the amount of weight the tire can safely support under the tire's maximum inflation pressure. So if a car requires a P195/75R14, changing to a P185/75R14 (smaller section width, and therefore less air capacity) or a P195/70R14 (lower aspect ratio, and therefore less air capacity) will be undersizing the tire, which risks premature tire wear or more serious tire failure due to the overloading. Changing to a lower aspect ratio, neverthe- less, will generally improve the handling char- acteristics of the vehicle. With a wider tread, the car will be more stable, will respond more quickly to steering, and will generally give you a better "feeling" of the road, as well as increasing traction when starting, stopping, or going around corners. Depending on tire design, however, switching to a lower aspect ratio may bring a harsher ride, as the shorter sidewall will absorb less road shock. To avoid undersizing when changing to a lower aspect ratio, you have to increase the sec- tion width to compensate for the decrease in sidewall height. Using the example above, you'd have to substitute a P205/70R14, or larg- er, to keep the same, or greater, load carrying capacity. Manufacturers often recommend an optional size for the vehicle. The Tire Guide, published by Bennett Garfield, which any competent tire dealer should have, lists virtually all cars and light trucks and their standard and approved optional tire sizes. The Tire Guide and a com- petent salesman can advise you of your options. Many cars can use tires at least one size over the standard size; too large, though, and you risk having clearance problems with the body of the car. In addition to increased handling and trac- tion, larger tires can provide an increase in ride comfort, because there is more tire to absorb any road shock from bumps, potholes, and other road irregularities. Also, a larger tire does not have to work as hard as a smaller one, and thus should last longer, all other factors being equal. Changing tire size, however, can affect the speedometer reading. A taller tire than original will cause the meter to read slower than the car is actually traveling. Further, in newer, com- puter-controlled cars-which receive informa- tion directly from the speedometer-meter error can cause the car to run rough, erratic, or not at all. Check with your local car dealer ser- vice department to see if such a change will affect your vehicle. Finally, whenever possible, do not mix tire sizes on the same vehicle. Deviation from uni- form sizing of all tires can cause erratic han- dling. Tire Type Very important to the safety, longevity, and ride of any particular tire is its construction. Generally, there are three types of tires on the road today: Diagonal Ply, or Bias, Tires. This is the oldest and simplest form of tire construction, but does not make for a very long-lasting tire. It is called diagonal because the fabric that makes up the tire overlaps itself in layers as the tire is put together; the layers run in diagonal lines across the tire. Because the layers overlap, they tend to rub against each other, causing friction and heat buildup as the tire moves down the road. The more friction and heat, the faster the tire will wear out. Also, with a bias design, there is no rein- forcement under the tread to keep the tread flat on the ground at all times. As the tire rotates, the tread face itself moves around on the road and wears down quickly. This exces- sive tread movement, or squirm, also reduces traction and stability because the tread is not always in full contact with the pavement. Diagonal ply tires represent the least expen- sive products on the market, but should never- theless be viewed in relation to their perfor- mance characteristics. Bias Belted. These, simply, are diagonal ply tires with a reinforcing belt under the tread. The belt helps keep the tread more stable and rigid, so that it stays in contact with the road- providing better traction and stability than conventional diagonal ply tires. Radial Tires. This is the latest, most popular method of tire construction, so-called because the fibers that make up the body of the tire run radially-at a 900 angle to the bead, or edge, from bead to bead. This method of construction gives less rolling resistance because the plies are not rubbing against each other, as in a bias tire. Radials always contain some sort of belt under the tread, which increases their effec- tiveness. Compared with other bias or bias-belt- ed construction, a radial offers several advan- July 1991 17

tages, including: less rolling resistance for increased fuel economy; less friction and heat buildup for longer tire life; easier rolling for smoother, quieter ride; and less tread "squirm" for better traction and wear. Tread Design Radials also offer more versatility in tread designs, offering "rib," "snow," and "all-sea- son" varieties. Rib. The rib design represents a tread that has several straight grooves running along the whole tread of the tire. Because of its simple tread design, it generally wears very evenly. Snow. The snow tread was designed because a rib tire did not have the traction that was need- ed for slippery ice and snow conditions. These tires have heavy, open lugs on the shoulder, and widely spaced tread patterns that clean themselves of snow and slush. The shoulder lugs bite into the ice and snow very well, but What About Uniform Tire Grading? Uniform Tire Quality Grading ratings are molded into the rubber on the side of pas- senger tires along with the other informa- tion on tire sidewalls. These ratings denote treadwear, traction, and temperature test results of the tire. The system was designed by the National Highway Traffic Safety Administration to rate the performance of tires so consumers could make informed decisions when purchasing them. The objec- tive is a good one, but, unfortunately, it has generated confusion and controversy, though not much help for purchasing deci- sions. The treadwear number is a comparative numerical rating, giving an indicator of how long a tire will last. For example, a tire rated "150" would theoretically wear 50% better than a tire rated "100." The testing is performed on a government-specified course in Texas, on which, basically, a con- voy of cars fitted with both test and control tires run on a 400-mile course, first break- ing in the test tires for 800 miles, then actu- ally testing the tires for another 6,200 miles. During the testing, the tires are rotated, front-end alignments are rechecked every 800 miles, and the tires are allowed to cool. From this, the treadwear rating is determined. This rating does not indicate "real world" tire mileage for a number of reasons. For example, nobody rotates their tires and checks alignment every 800 miles. Also, drivers of test cars drive under certain restrictions, such as having to slow down a certain distance before a stop sign. And, tires tested on a rear-wheel drive car could wear differently from those mounted on a front-wheel drive. Finally, in 7,200 miles, under. ideal conditions, treadwear can be 18 Consumers' Research minimal, or nonexistent-leaving the door open for inexact ratings by manufacturers. The rating might give an indicator of differ- ences within a specific manufacturer's tire line, but does not, for the most part, offer a good basis of comparison between different brands. The traction grades appear as A, B, or C (from highest to lowest) next to the tread- wear rating. In the traction test, a tire is attached to a trailer and locked on a wet surface at a specified speed. The friction generated by the tire is measured and, from this, the rating is assigned. This tests for one condition only-stop- ping on wet pavement. Snow and ice trac- tion, cornering traction, dry traction, accel- eration traction, and hydroplaning are not indicated in the rating. A tire with the low mark of C might be excellent in snow, or cornering; a tire rated A might be relatively poor under these conditions. Following traction is the temperature rat- ing, also denoted as, from highest to lowest, A, B, or C. Under the temperature test, a tire is pressed against a revolving drum to simulate a specific load. The tire is spun at a certain speed for a certain length of time. All tires must pass minimum requirements to be allowed on the market for sale. These minimum requirements are denoted by the C rating and represent the tire's ability to dissipate and withstand heat at a sustained speed of 85 mph. The ratings B and A repre- sent tests at higher sustained speeds of 100 and 115 mph, respectively. This is the most straightforward test of the three, and gives a more realistic indica- tor of a tire's performance. Still, keep in mind that even the C rating represents test speeds rarely seen by most drivers.