Behavior of new type of rock wool (HT wool) in lungs after exposure by nasal inhalation in rats

Dissolution behavior of stone wool fibers in synthetic lung fluids: Impact of iron oxidation state changes induced by heat treatment for binder removal

Stone wool fiber materials are commonly used for thermal and acoustic insulation, horticulture and filler purposes. Biosolubility of the stone wool fiber (SWF) materials accessed through acellular in vitro dissolution tests can potentially be used in future as an indicator of fiber biopersistence in vivo. To correlate acellular in vitro studies with in vivo and epidemiological investigations, not only a robust dissolution procedure is needed, but fundamental understanding of fiber behavior during sample preparation and dissolution is required. We investigated the influence of heat treatment procedure for binder removal on the SWF iron oxidation state as well as on the SWF dissolution behavior in simulant lung fluids (with and without complexing agents). We used heat treatments at 450 °C for 5 min and 590 °C for 1 h. Both procedures resulted in complete binder removal from the SWF. Changes of iron oxidation state were moderate if binder was removed at 450 °C for 5 min, and there were no substantial changes of SWF's dissolution behavior in all investigated fluids after this heat treatment. In contrast, if binder was removed at 590 °C for 1 h, complete Fe(II) oxidation to Fe(III) was observed and significant increase of dissolution was shown in fluids without complexing agent (citrate). PHREEQC solution speciation modeling showed that in this case, released Fe(III) may form ferrihydrite precipitate in the solution. Precipitation of ferrihydrite solid phase leads to removal of iron cations from the solution, thus shifting reaction towards the dissolution products and increasing total mass loss of fiber samples. This effect is not observed for heat treated fibers if citrate is present in the fluid, because Fe(III) binds with citrate and remains mobile in the solution. Therefore, for developing the most accurate SWF in vitro acellular biosolubility test, SWF heat treatment for binder removal is not recommended in combination with dissolution testing in fluids without citrate as a complexing agent.

Comment on "Which fraction of stone wool fibre surface remains uncoated by binder? A detailed analysis by time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy" by Hirth et al., 2021, RSC Adv., , 39545, DOI: 10.1039/d1ra06251d

The article mentioned in the title of this comment paper reports on an investigation of the organic binder presence and distribution on stone wool fibres with surface sensitive techniques (X-ray photoelectron spectroscopy (XPS), QUASES XPS modelling, time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping) and attempts to correlate the results with fibre performance in in vitro acellular biosolubility tests. However, the study has assumptions, hypothesis and results that do not take into account the recognised science and regulations on biopersistence of stone wool fibres, limitations of the utilized surface sensitive techniques and modelling approach and it contains a contradiction with biosolubility experiments. In this comment article, we discuss these points, propose improved QUASES XPS modelling and present recent ToF-SIMS mapping results that reflect biosolubility behaviour of the stone wool fibres.

The dissolution of stone wool fibers with sugar-based binder and oil in different synthetic lung fluids

The biopersistence of fiber materials is one of the cornerstones in estimating potential risk to human health upon inhalation. To connect epidemiological and in vivo investigations with in vitro studies, reliable and robust methods of fiber biopersistence determination and understanding of fiber dissolution mechanism are required. We investigated dissolution properties of oil treated stone wool fibers with and without sugar-based binder (SBB) at 37 °C in the liquids representing macrophages intracellular conditions (pH 4.5). Conditions varied from batch to flow of different rates. Fiber morphology and surface chemistry changes caused by dissolution were monitored with scanning electron microscopy and time-of-flight secondary ion mass spectrometry mapping. Stone wool fiber dissolution rate depends on liquid composition (presence of ligands, such as citrate), pH, reaction products transport and fibers wetting properties. The dissolution rate decreases when: 1) citrate is consumed by the reaction with the released Al cations; 2) the pH increases during a reaction in poorly buffered solutions; 3) the dissolution products are accumulated; 4) fibers are not fully wetted with the fluid. Presence of SBB has no influence on dissolution rate if fiber material was wetted prior to dissolution experiment to avoid poorly wetted fiber agglomerates formation in the synthetic lung fluids.

Insulation fiber deposition in the airways of men and rats. A review of experimental and computational studies

The typical insulation rock, slag and glass wool fibers are high volume materials. Current exposure levels in industry (generally ≤ 1 fiber/cm³ with a median diameter ∼1 μm and length ≥10 μm) are not considered carcinogenic or causing other types of severe lung effects. However, epidemiological studies are not informative on effects in humans at fiber levels >1 fiber/cm³. Effects may be inferred from valid rat studies, conducted with rat respirable fibers (diameter ≤ 1.5 μm). Therefore, we estimate delivery and deposition in human and rat airways of the industrial fibers. The deposition fractions in humans head regions by nasal (∼0.20) and by mouth breathing (≤0.08) are lower than in rats (0.50). The delivered dose into the lungs per unit lung surface area during a 1-day exposure at a similar air concentration is estimated to be about two times higher in humans than in rats. The deposition fractions in human lungs by nasal (∼0.20) and by mouth breathing (∼0.40) are higher than in rats (∼0.04). The human lung deposition may be up to three times by nasal breathing and up to six times higher by oral breathing than in rats, qualifying assessment factor setting for deposition.

Magnetometric evaluation of toxicities of chemicals to the lungs and cells

Because the lungs are exposed to airborne hazardous materials, alveolar macrophages (AMs) play a major role in defending against the exposure to various noxious chemical substances. In this study, we reviewed magnetometric investigations of the effects of various chemicals on the lungs and AMs. Magnetometry, using magnetite as an indicator, was used to evaluate the effects of certain chemicals on the lung and AMs. A rapid decrease of the remanent magnetic field after the cessation of external magnetization, a phenomenon called relaxation, was impaired when the lungs and macrophages were exposed to toxic substances. The delayed in vivo relaxation observed in the lungs exposed to magnetite and gallium arsenide was almost identical to the in vitro relaxation observed in the AMs exposed to the same materials. Delayed relaxation was observed in the AMs exposed to silica dust; various fibers, such as chrysotile and some man-made mineral fibers; and toxic arsenic and cadmium compounds. The extracellular release of lactate dehydrogenase activity was found in the AMs exposed to the chemicals. Relaxation is attributed to the cytoskeleton-driven rotation of phagosomes containing magnetite. While the exact mechanism of delayed relaxation due to exposure to harmful chemicals remains to be clarified, cell magnetometry appears to be useful for the safety screening of chemical substances.

Behavior of rock wool in lungs after exposure by nasal inhalation in rats

To evaluate the safety of rock wool (RW fibers), we examined the biopersistence of a RW sample in the lungs of rats, based on the changes of fiber number and fiber size in terms of length and width, by a nose-only inhalation exposure study. Twenty male Fischer 344 rats (6-10 weeks old) were exposed to RW fibers at a concentration of 70 (21) fiber/m(3) and 30 (6.6) mg/m(3), arithmetic mean (geometric standard deviation), continuously for 3 h daily for five consecutive days. Five rats each were sacrificed shortly and at 1, 2, and 4 weeks after exposure, and their lung tissues were ashed by a low-temperature plasma-asher. Then, the numbers and sizes of fibers in the ashed samples were determined using phase-contrast microscope and computed image analyzer. The fiber numbers in the lungs 4 weeks after exposure significantly decreased from the baseline value, i.e., shortly after exposure (P < 0.05). The half-lives of RW fibers calculated from the one-compartment model were 32 days for total fibers and 10 days for fibers longer than 20 mum. The decrease of fiber number was 53.6% by 4 weeks after exposure (baseline group = 100%). Likewise, fiber sizes significantly decreased by 4 weeks after exposure (P < 0.05), probably because fibers were dissolved in body fluid, ingested by alveolar macrophages or discharged to outside of the body by mucociliary movement. In future studies, it is necessary to examine the long-term persistence of RW fibers in the lungs.

Effects of rock wool on the lungs evaluated by magnetometry and biopersistence test

BACKGROUND

Asbestos has been reported to cause pulmonary fibrosis, and its use has been banned all over the world. The related industries are facing an urgent need to develop a safer fibrous substance. Rock wool (RW), a kind of asbestos substitute, is widely used in the construction industry. In order to evaluate the safety of RW, we performed a nose-only inhalation exposure study in rats. After one-month observation period, the potential of RW fibers to cause pulmonary toxicity was evaluated based on lung magnetometry findings, pulmonary biopersistence, and pneumopathology.

METHODS

Using the nose-only inhalation exposure system, 6 male Fischer 344 rats (6 to 10 weeks old) were exposed to RW fibers at a target fiber concentration of 100 fibers/cm3 (length [L] > 20 mum) for 6 hours daily, for 5 consecutive days. As a magnetometric indicator, 3 mg of triiron tetraoxide suspended in 0.2 mL of physiological saline was intratracheally administered after RW exposure to these rats and 6 unexposed rats (controls). During one second magnetization in 50 mT external magnetic field, all magnetic particles were aligned, and immediately afterwards the strength of their remanent magnetic field in the rat lungs was measured in both groups. Magnetization and measurement of the decay (relaxation) of this remanent magnetic field was performed over 40 minutes on 1, 3, 14, and 28 days after RW exposure, and reflected cytoskeleton dependent intracellular transport within macrophages in the lung. Similarly, 24 and 12 male Fisher 344-rats were used for biopersistence test and pathologic evaluation, respectively.

RESULTS

In the lung magnetometric evaluation, biopersistence test and pathological evaluation, the arithmetic mean value of the total fiber concentration was 650.2, 344.7 and 390.7 fibers/cm3, respectively, and 156.6, 93.1 and 95.0 fibers/cm3 for fibers with L > 20 mum, respectively. The lung magnetometric evaluation revealed that impaired relaxation indicating cytoskeletal toxicity did not occur in the RW exposure group. In addition, clearance of the magnetic tracer particles was not significantly affected by the RW exposure. No effects on lung pathology were noted after RW exposure.

CONCLUSION

These findings indicate that RW exposure is unlikely to cause pulmonary toxicity within four weeks period. Lung magnetometry studies involving long-term exposure and observation will be necessary to ensure the safety of RW.

Biopersistence of rock wool in lungs after short-term inhalation in rats

To evaluate the safety of rock wool (RW), an asbestos substitute, we examined the biopersistence of RW fibers in rat lungs based on the changes of fiber number and fiber size (length and diameter) by a nose-only inhalation exposure study. Twenty-four male Fischer 344 rats were exposed to RW fibers at a concentration of 30 mg/m(3) continuously for 3 h daily for 5 consecutive days. Six rats each were sacrificed shortly and at 1, 2, and 4 wk after exposure, and their lung tissues were ashed by a low-temperature plasma asher. Then the fiber numbers and fiber sizes in lungs were determined using a phase-contrast microscope and computed image analyzer. During the study period, the arithmetic mean (SD) values of fiber and weight concentrations were 78.5 (35.7) fibers/cm(3), and 29.9 (28.3) mg/m(3), respectively. The fiber number in lungs 4 wk after exposure significantly decreased from the baseline value (shortly after exposure) (p < .05). The half-life of fibers calculated from the approximate curve was 28 days for all fibers and 16 days for fibers with L > 20 microm, and the rate of decrease in fiber number was 46.3% at 4 wk after exposure (shortly-after group = 100%). Likewise, both length and diameter significantly decreased at 4 wk after exposure (p < .05), probably because fibers were phagosytosed and digested by alveolar macrophages, discharged to outside of the body by mucociliary movement, or dissolved by body fluid. It will be necessary in the future to further confirm the safety of RW fibers by assessing the biopersistence of fibers in the lungs and their pathological effects in our ongoing study performed in accordance with the guidelines established in the "Methods for Determination of Hazardous Properties for Human Health of Man Made Mineral Fibers" (EC protocol).

Behavior of rock wool in rat lungs after exposure by nasal inhalation

To evaluate the safety of rock wool (RW) fibers, we examined the biopersistence of RW fibers in the lungs of rats, based on the changes of fiber number and fiber size in the length and width, in a nose-only inhalation exposure study. Twenty male Fischer 344 rats (6 to 10 wk old) were exposed to RW fibers at a fiber concentration of 70.6 (20.4) fiber/m(3) and a dispersion density of 30.4 (6.6) mg/m(3) [arithmetic mean (SD)] continuously for 3 h daily for 5 consecutive days. Five rats each were sacrificed shortly after exposure ended (baseline group) and at 1, 2, and 4 wk after exposure, and their lung tissues were ashed by a low temperature plasma-asher. The numbers and sizes of fibers in the ash samples were determined using a phase contrast microscope and a computed image analyzer. The fiber numbers in the lungs at 4 wk after exposure had significantly decreased from the baseline value, i. e. shortly after exposure (p<0.05). The half-lives of RW fibers calculated using the one-compartment model were 32 d for total fibers and 10 d for fibers longer than 20 microm in length. Fiber number was 53.6% of the baseline at 4 wk after exposure (baseline group=100%). Likewise, fiber sizes had significantly decreased at 4 wk after exposure (p<0.05), probably because fibers had been dissolved in body fluid, phagocytosed by alveolar macrophages or discharged from the body by mucociliary movement. In future studies, it will be necessary to examine the carcinogenicity of RW fibers through long-term inhalation studies.