Threshold Limit Values (TLVs) represent conditions under which the TLV Committee of the American Conference of Governmental Industrial Hygienists (ACGIH) believes that nearly all workers may be repeatedly exposed without adverse effect. A detailed research was made of the references in the 1976 Documentation to data on "industrial experience" and "experimental human studies." The references, sorted for those including both the incidence of adverse effects and the corresponding exposure, yielded 158 paired sets of data. Upon analysis it was found that, where the exposure was at or below the TLV, only a minority of studies showed no adverse effects (11 instances) and the remainder indicated that up to 100% of those exposed had been affected (8 instances of 100%). Although, the TLVs were poorly correlated with the incidence of adverse effects, a surprisingly strong correlation was found between the TLVs and the exposures reported in the corresponding studies cited in the Documentation. Upon repeating the search of references to human experience, at or below the TLVs, listed in the more recent, 1986 edition of the Documentation, a very similar picture has emerged from the 72 sets of clear data which were found. Again, only a minority of studies showed no adverse effects and TLVs were poorly correlated with the incidence of adverse effect and well correlated with the measured exposure. Finally, a careful analysis revealed that authors' conclusions in the references (cited in the 1976 Documentation) regarding exposure-response relationships at or below the TLVs were generally found to be at odds with the conclusions of the TLV Committee. These findings suggest that those TLVs which are justified on the basis of "industrial experience" are not based purely upon health considerations. Rather, those TLVs appear to reflect the levels of exposure which were perceived at the time to be achievable in industry. Thus, ACGIH TLVs may represent guides of levels which have been achieved, but they are certainly not thresholds.

The prevalence of byssinosis was measured in a population of 189 male and 780 female workers employed in three coarse and two fine cotton mills. Ninety-eight per cent. of the male and 96% of the female population were seen.The workers were graded by their histories as follows: Grade 0-No symptoms of chest tightness or breathlessness on MondaysGrade I-Occasional chest tightness on Mondays, or mild symptoms such as irritation of the respiratory tract on Mondays Grade 1-Chest tightness and/or breathlessness on Mondays only Grade 2-Chest tightness and/or breathlessness on Mondays and other days The dust concentrations to which the workers were exposed were measured with a dust-sampling instrument based on the hexhlet. Altogether 505 working places were sampled. In the card-rooms of the coarse mills 63 % of the men and 48 % of the women had symptoms of byssinosis. In the card-rooms of the fine mills the corresponding prevalences were 7 % for the men, and 6% for the women. Prevalences were low in the spinning-rooms in the coarse mills. The mean dust concentrations in the different rooms ranged from 90 mg./100 m.3 in one section of the card-room in a fine mill, to 440 mg./100 m.3 in one of the card-rooms of the coarse spinning mills. The prevalence of byssinosis in the different rooms was closely related to the overall dustiness (r = 0-93). For the three main constituents of the dust, namely, cellulose, protein, and ash, the prevalence of byssinosis correlated most highly with protein, particularly with the protein in the medium-sized dust particles, i.e., approximately 7 microns to 2 mm.

Studies of ventilatory capacity change in small groups of employees during a shift in a cotton mill and in three cotton ginneries in Uganda, a sisal factory in Kenya, and a jute mill in England, have demonstrated that an effect is produced by the dust in the cotton mill and in a very dusty ginnery but not in two other less dusty ginneries. No significant effect was detected in the sisal factory or in the jute mill despite much higher dust concentrations than in the cotton mill.The dust sampling instruments gave the weight in three sizes: Coarse (>2 mm.), medium (7,u to 2 mm.), and fine (< 7,). The samples were analysed for protein, mineral (ash), and cellulose (by difference). The fine and medium sisal and jute dusts contain less protein than cotton dusts. The physiological changes observed in the employees in the cotton mill indicate the need for general dust measurement and control, even when new carding machinery is installed in a new mill.The ventilatory capacity of normal subjects and of those with byssinosis falls during the course of a day's exposure to the dust in cotton mill card-rooms (McKerrow, McDermott, Gilson, and Schilling, 1958; Bouhuys, Lindell, and Lundin, 1960). The agent producing this effect has not yet been isolated. Davenport and Paton (1962) have shown that cotton dust extracts contain a smooth muscle contractor substance which may be responsible. Antweiler (1960) believes that the agent is a histamine releaser, and Pernis, Vigliani, Cavagna, and Finulli (1961) have suggested that the effect may be due to bacterial endotoxins. Tuffnell (1960) suggests that the substance is in the bracts and pericarp of the cotton boll; this is supported by Nicholls (1962) who has confirmed the presence of a smooth muscle contractor substance in extracts of these parts of the plant and has shown that it caused contraction of isolated human bronchial muscle. Nicholls has also investigated extracts of dust from flax, sisal, and jute mills and found decreasing activity in this order.Here we report complementary studies of the Present addresses:

A group of 379 men who had worked at an asbestos textile factory for at least 10 years has been followed up. The prevalence of crepitations, 'possible asbestosis', certified asbestosis, small opacities in the chest radiograph and values of lung function have been related to dust levels. The type of asbestos processed was predominantly chrysotile although a substantial amount of crocidolite had also been used in the past. There was a higher prevalence of crepitations than had been observed previously at the same factory. The presence of crepitations is not a specific effect of asbestos exposure and 'possible asbestosis', a combined judgement of two physicians on whether a man had developed signs which might be attributable to early asbestosis, was preferred. Fifty per cent ofmen with a diagnosis of possible asbestosis were certified as suffering from asbestosis by the Pneumoconiosis Medical Panel within 3-5 yr. The most reliable data relate to men first employed after 1950; 6 6"/o of men in this group had possible asbestosis after an average length of follow-up of 16 yr and an average exposure to 5 fibre/cm3 where the dust levels were determined by static area samplers. The forced expiratory volume and forced vital capacity declined significantly with exposure, after allowing for age and height, but there was no decline in the total lung capacity. The transfer factor also declined with exposure, but not to a statistically significant extent. The non-smokers and light smokers as a group had less crepitations, asbestosis and small opacities on the chest radiograph than heavier smokers with similar exposure. Combining dust concentrations to form the cumulative dose may not be completely satisfactory, and a family of measures was investigated which allows for elimination of dust from the lungs and includes the cumulative dose as a special case. Because the rate of elimination of dust from the lungs is unknown, and cannot be estimated from the data, this approach leads to a wide range of possible interpretations of the data; for example the concentration such that possible asbestosis occurs in no more than 1 % of men after 40 years' exposure could be as high as 1 1 fibres/cm3 or may have to be as low as 0-3 fibres/cm3. This range is wide because the data relate to higher dust levels, and a shorter period of follow-up. Until data are available on groups exposed to lower levels it will not be possible to assess the effects of the current standard with any certainty. However, the results of this study show that it is important to continue to reduce dust levels to values as low as possible.In 1968 the British Occupational Hygiene Society centration x period of exposure to that concentra-(BOHS) published hygiene standards for chrysotile tion over the whole period of exposure. With an asbestos dust (British Occupational Hygiene Society, accumulated exposure of 100 fibre/years/cm3 it was 1968). One of thc features was that risk was related concluded that it was probable that the risk of conto accumulated exposure, t...

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