HP1 reshapes nucleosome core to promote phase separation of heterochromatin.

Heterochromatin affects genome function at many levels. It enables heritable gene repression, maintains chromosome integrity and provides mechanical rigidity to the nucleus1,2. These diverse functions are proposed to arise in part from compaction of the underlying chromatin2. A major type of heterochromatin contains at its core the complex formed between HP1 proteins and chromatin that is methylated on histone H3, lysine 9 (H3K9me). HP1 is proposed to use oligomerization to compact chromatin into phase-separated condensates3-6. Yet, how HP1-mediated phase separation relates to chromatin compaction remains unclear. Here we show that chromatin compaction by the Schizosaccharomyces pombe HP1 protein Swi6 results in phase-separated liquid condensates. Unexpectedly, we find that Swi6 substantially increases the accessibility and dynamics of buried histone residues within a nucleosome. Restraining these dynamics impairs compaction of chromatin into liquid droplets by Swi6. Our results indicate that Swi6 couples its oligomerization to the phase separation of chromatin by a counterintuitive mechanism, namely the dynamic exposure of buried nucleosomal regions. We propose that such reshaping of the octamer core by Swi6 increases opportunities for multivalent interactions between nucleosomes, thereby promoting phase separation. This mechanism may more generally drive chromatin organization beyond heterochromatin.

A comparison of the sampling efficiencies of a range of atmosphere samplers when collecting polymeric diphenylmethane di-isocyanate (MDI) aerosols.

Polymeric diphenylmethane di-isocyanate (PMDI) is a precursor or an intermediate in the manufacture of pure MDI and is used in many industrial applications. Potential health effects of diisocyanates are generally considered to include irritation and respiratory sensitization, dictating the need for low occupational hygiene standards and robust hygiene monitoring methods. A wide range of methods has been developed or adopted for this purpose but questions concerning their ability to accurately sample MDI aerosols in the occupational environment have often been raised. In order to provide such information, studies have been conducted to compare several MDI sampling systems: the Institute of Occupational Medicine (IOM) 25-mm open-faced inhalable dust sampler; the 13-mm Millipore Swinnex Cassette sampler; the 37-mm open-face Millipore cassette; the midget-impinger, the glass tube containing glass wool, and two direct reading paper tape monitors. The program was comprised of two phases, the first being a preliminary comparison of the collection efficiencies of the IOM, 13 mm and midget-impinger at a range of orientations to air flow, aerosol particle sizes, and sampling flow rates, using inert polyethylene glycol aerosols. The second phase compared all samplers operating according to each manufacturer's recommendations and sampling PMDI aerosols at a range of particle sizes and concentrations. All studies were conducted in a wind tunnel. All filter methods performed well in atmospheres containing small particles except impingers that required a filter backup. In general, the variability of all the samplers was high for larger particle size ranges. Direct reading monitors showed low efficiencies.

Evaluation of organic-vapor respirator cartridge efficiency for hexamethylene diisocyanate vapor in the presence of organic solvents.

Hexamethylene diisocyanate (HDI)-based polyisocyanates are widely used in formulating polyurethane coatings. These polyisocyanates contain a small amount of HDI monomer and the American Conference of Governmental Industrial Hygienists (ACGIH) recommends an eight-hour time-weighted average (TWA) threshold limit value (TLV) of 5 ppb for HDI. Some polyurethane (PU) paint applicators have used air-purifying paint spray respirators with organic vapor cartridges and particulate pre-filters. In this study, the effect of typical paint solvents on the efficiency of organic vapor cartridges (OVC) for HDI vapor was tested. A Teflon-coated polycarbonate exposure chamber was constructed. Three OVCs were simultaneously tested in an atmosphere containing HDI vapor and solvents. The test atmosphere was generated by evaporating a mixture of paint solvents containing n-butyl acetate, propylene glycol mono methyl ether acetate, toluene and methyl ethyl ketone, and neat HDI. The target challenge concentrations were 20 times the TLV for HDI and 10 times the combined TLV for the solvent mixture. The test atmosphere, with 20 or 80 percent relative humidity and at room temperature, was drawn through each cartridge at 32 L/min for 40 hours. However, in the last 8 hours of the test, the atmosphere had only HDI vapor. The pre- and the post-cartridge atmospheres were periodically sampled for HDI and the solvents. The study concluded that under the test conditions there was no detectable breakthrough of HDI from the OVCs. The average calculated efficiency of the OVCs, based on the HDI analytical limit of quantitation, was >99.4 percent for the 40 hours tested. The overall average challenge concentration of HDI was 105 ppb and the average combined solvent concentration was 3,176 mg/m3. The cartridges were saturated with solvents within the first 10 hours of testing; nevertheless, continued challenging with HDI vapor and solvents did not show any HDI breakthrough. Solvent breakthrough and breathing resistance, rather than HDI breakthrough, would be the key parameters in life expectancy calculations for cartridge change out schedules.

A survey of airborne HDI, HDI-based polyisocyanate and solvent concentrations in the manufacture and application of polyurethane coatings.

This study summarizes the results of industrial hygiene surveys performed between 1979 and 1987 in paint manufacturing and application operations using polyurethane coatings containing hexamethylene diisocyanate (HDI) and HDI-based polyisocyanates (trade name Desmodur N). A total of 466 Desmodur N and 457 HDI samples were collected from 47 operations most of which were in application. The application surveys covered manufacture and refinishing of transportation vehicles, painting of large military and civilian equipment, industrial finishing operations, and maintenance and construction operations. The primary objective of the surveys was to assess the potential exposure to HDI and HDI-based polyisocyanate. In more than 60% of the surveys, concentrations of airborne organic solvents also were monitored. Isocyanates were sampled using toluene/nitroreagent in midget impingers, and solvents were collected using charcoal tubes. They were analyzed using high pressure liquid chromatography and gas chromatography, respectively. The data from these workplace situations show there is some potential for isocyanate overexposure of unprotected workers and that it is greater in spray than in nonspray operations.

Evaluation of air-purifying respirators for protection against toluene diisocyanate vapors.

Two brands of air-purifying organic vapor cartridges (Willson and Survivair) and a disposable respirator (3M) were evaluated for protection against toluene diisocyanate (TDI) vapors. The respirators/cartridges were tested by generating dynamic atmospheres of TDI at concentrations of 0.2 and 1.5 ppm or greater, which are substantially above the currently accepted exposure limits. The TDI atmospheres were generated by controlled and continuous evaporation and dilution techniques. The relative humidity of the final TDI atmosphere was maintained at 50%. For the testing of Survivair and Willson respirators, one cartridge was mounted on a stainless steel plate and placed inside an exposure chamber through which air was drawn unidirectionally at 32 L/min. Periodically, the air before and after the cartridge was monitored for TDI. In the case of the disposable, valveless 3M respirator, a breathing pump was used to simulate the breathing through the respirator. The TDI atmosphere was respired through the respirator at 28.8 L/min (24 cycles/min at 1.2 L/cycle). As before, the concentration of TDI was measured periodically before and after the respirator. There was no significant breakthrough (less than 0.5%) of TDI in any of the respirators tested for 40 hr at 0.2 ppm and for 20 hr at 1.5 ppm or higher concentration of TDI. The detection limits of the post-respirator TDI measurements ranged from 0.4 to 0.02% of the pre-respirator concentration. It is important to note that, at the present time, because the odor threshold for TDI is higher than the ceiling exposure limit (poor warning property), NIOSH and most of the respirator manufacturers do not recommend the use of air-purifying respirators in isocyanate containing environments.

Five-year longitudinal study of workers employed in a new toluene diisocyanate manufacturing plant.

The respiratory health of 277 workers in a new toluene diisocyanate (TDI) manufacturing plant was studied prospectively during 5 yr of exposure. Personal TDI monitors were used to continuously measure peak and 8-h time-weighted average (TWA) concentrations in over 2,000 samples. Longitudinal change in pulmonary function was assessed in 223 men in whom 3 or more data points allowed construction of individual slopes of annual change. Regression of annual change on smoking (pack-years), atopic status, and cumulative TDI exposure dichotomized at 68.2 parts per billion (ppb) months into low and high exposure groups showed significant effects of smoking on spirometric tests and lung volumes. After adjusting for pack-years of smoking, the 74 men in the high cumulative TDI exposure category had significantly larger declines in FEV1, %FEV, and FEF25--75% than did the 149 men in the low category. Annual change in FEV1 was then examined in 6 smoking-exposure categories: in never smokers, average annual decline was 38 ml/yr greater in those with higher cumulative TDI exposure. Current and previous cigarette smokers did not show this effect of cumulative TDI exposure. Analysis of FEV1 change by time above 20 ppb TDI yielded a similar result in never smokers, a 24 ml/yr excess average decline attributable to longer time above 20 ppb. In current cigarette smokers, those with longer time above 20 ppb had excess decline of 18 ml/yr (42 versus 24 ml/yr). our low and high cumulative exposure groups spent 2 and 15%, respectively, of their working time above 5 ppb TDI. The different health effects observed in these groups supports the NIOSH-recommended standard of 5 ppb TDI as an 8-h TWA.

Critical evaluation of continuous monitors for toluene diisocyanate.

This paper describes the experiments and the results of an exhaustive and critical laboratory study which evaluated the performance of continuous area and personal monitors popularly used in measuring the concentrations of TDI in industrial environments. The studies consisted of dynamic calibrations, the effect of humidity and temperature on the measurements, and the interferences due to commonly encountered contaminants in a TDI manufacturing plant. The response and resolution of the personal monitor for short-term fluctuations of TDI concentration in time and space are also reported.

Dynamic calibration of a continuous organo-isocyanate monitor for hexamethylene diisocyanate.

In 1978, NIOSH recommended new exposure standards for all diisocyanates. They are 5 ppb TWA for a 10-hour work shift and 20 ppb ceiling for a 10-minute period (or the equivalent microgram/m3). Recently, a new continuous reading monitor, which responds immediately to both aliphatic and aromatic diisocyanates, has become commercially available. This instrument factory calibrated for measuring toluene diisocyanate, has different responses for different diisocyanates. The monitor was dynamically calibrated to measure hexamethylene diisocyanate (HDI) against the recommended high pressure liquid chromatographic methods. The standard atmospheres of HDI were generated using permeation and an evaporation method. The monitor reading is 50% of the true concentration up to 50 ppb. Beyond that, the monitor reading reaches a plateau and the measured concentrations could be unreliable.

Mill effect and dose-response relationships in byssinosis.

Four hundred and eighty-six textile workers in three cotton mills and one wool/synthetic mill were studied for symptoms and functional effects of workroom exposure to dust. Byssinosis was found in 5.7% of 386 cotton workers, with an apparent threshold level of 0.5 mg cotton dust/m3 of air. Mean post-shift functional declines were greater in workers exposed to greater than or equal to 0.2 mg/m3. Workers with byssinosis were unequally distributed, however, with respect to job category and mill; and these variables, rather than current dust exposure levels, accounted for the observed distribution of byssinosis prevalence rates. Variation in biological potency of different samples of cotton dust could be responsible for 'mill effect', the residual variation in response rates by mill after controlling for variation due to dust exposure. A number of other potential influencing variables that are likely to be distributed unequally by mill should also be considered. Mill effect should be assessed in large-scale studies of byssinosis, most of which have analysed biological response rates by combining mill and other variables to examine first-order effects of dust dosage. In such analyses, much of the observed variability may be due to factors other than dust dosage.

A new method for the generation of standard atmospheres of organo isocyanates.

A new and versatile method for the generation of standard atmospheres of several organo isocyanates based on permeation technique has been developed. Several membranes were screened for integrity, compatibility and permeability of these isocyanates. Silicone membrane was found to be the best membrane for the construction of permeation devices. Two types of permeation are described: liquid phase and gas phase. The liquid phase permeation tubes are recommended for generation of high concentrations (ppm) and gas phase permeation is preferred for generation of atmospheres with low concentrations (ppb). Several simple designs for construction of permeation devices are described. The permeation of isocynates were evaluated gravimetrically and were shown to be constant, provided the temperature and the flow rate of air over the permeation tube are maintained constant.

Inhalation challenge and pharmacologic studies of toluene diisocyanate (TDI)-sensitive workers.

Workers with "sensitivity" to toluene diisocyanate (TDI) studied in depth in an attempt to determine mechanisms of bronchial hyperreactivity. Tests included provocative inhalation challenge (PIC) with TDI and methacholine challenge. Blood samples obtained prior to and at various times after PIC were used to measure complement and split products of complement and plasma histamine levels and to determine dose-response slopes of lymphocyte cyclic adenosine monophosphate (cAMP) following stimulation with agonists. TDI-reactive individuals were all reactive to methacholine and responded to PIC with TDI by immediate, delayed, or dual bronchospastic reactions. No change in plasma histamine, total complement levels, or split products of complement were measurable. TDI reactors gave decreased lymphocyte cAMP dose response slopes to stimulation with isoproterenol, prostaglandin E1, and TDI, which suggests that impairment of adrenergic receptors may play an important role in TDI reactivity.

Physical state of airborne p, p'-diphenylmethane diisocyanate (MDI) and its measurement.

The current exposure standard of 0.02 ppm ceiling for MDI by definition refers to a gaseous species. The standard method, recommended by NIOSH, for measuring MDI in air, is also based on the the assumption that MDI is in the gas phase. Investigations in our Unit indicate that a major portion of airborne MDI, in working environments during spraying operations, is present as an aerosol. In view of this finding, the standard method would underestimate the MDI concentration and describing the airborne MDI concentration in "ppm" or "volume/volume unit" in invalid. A new method for calibration of the continuous reading monitors to measure airborne MDI concentration in mg/m3 unit is described.

Longitudinal study of workers employed in the manufacture of toluene-diisocyanate.

Workers at a toluene-diisocyanate manufacturing plant were studied longitudinally to determine the effects of the chemical on their health. Studies included health questionnaire, pulmonary function, environmental monitoring, and immunologic testing. Workers reporting increased lower respiratory symptoms were from the nonsmoker group. Environmental monitoring showed frequent excursions of toluene-diisocyanate concentrations above the threshold limiting value. There was poor correlation between area and personal exposure levels. No exposure-related decline of pulmonary function was demonstrable. Immunologic studies showed development of a positive skin test to a toluene-diisocyanate-human serum albumin conjugate by some persons and an increasing incidence of toluene-diisocyanate-specific IgE antibodies as measured by a radioallergosorbent test. Toluene-diisocyanate did not induce histamine release from leukocytes in vitro but did diminish the in vitro stimulation of cyclic adenosine monophosphate by isoproterenol. Most of the clinically sensitive persons demonstrated adverse bronchial response when challenged by inhalation of toluene-diisocyanate. This response was dose dependent in some persons. When challenged with Mecholyl, clinically sensitive persons showed greater reactivity of airways than nonsensitive persons.