(a) Purpose. In the assessment and evaluation of the toxic characteristics of a gas, volatile substance, or aerosol/particulate, determination of subchronic inhalation toxicity may be carried out after initial information on toxicity has been obtained by acute testing. The subchronic inhalation study has been designed to permit the determination of the no-observed-effect level and toxic effects associated with continuous or repeated exposure to a test substance for a period of 90 days. The test is not capable of determining those effects that have a long latency period for development (e.g., carcinogenicity and life shortening). It provides information on health hazards likely to arise from repeated exposures by the inhalation route over a limited period of time. It will provide information on target organs, the possibilities of accumulation, and can be of use in selecting dose levels for chronic studies and for establishing safety criteria for human exposure. Hazards of inhaled substances are influenced by the inherent toxicity and by physical factors such as volatility and particle size.
(b) Definitions.
(1) Subchronic inhalation toxicity is the adverse effects occurring as a result of the repeated daily exposure of experimental animals to a chemical by inhalation for part (approximately 10 percent) of a life span.
(2) Aerodynamic diameter applies to the size of particles of aerosols. It is the diameter of a sphere of unit density which behaves aerodynamically as the particle of the test substance. It is used to compare particles of different size and densities and to predict where in the respiratory tract such particles may be deposited. This term is used in contrast to measured or geometric diameter which is representative of actual diameters which in themselves cannot be related to deposition within the respiratory tract.
(3) The geometric mean diameter or the median diameter is the calculated aerodynamic diameter which divides the particles of an aerosol in half based on the weight of the particles. Fifty percent of the particles by weight will be larger than the median diameter and 50 percent of the particles will be smaller than the median diameter. The median diameter describes the particle size distribution of any aerosol based on the weight and size of the particles.
(4) Inhalable diameter refers to that aerodynamic diameter of a particle which is considered to be inhalable for the organism. It is used to refer to particles which are capable of being inhaled and may be deposited anywhere within the respiratory tract from the trachea to the alveoli. For man, inhalable diameter is considered as 15 micrometers or less.
(5) Dose refers to an exposure level. Exposure is expressed as weight or volume of test substance per volume of air (mg/l), or as parts per million (ppm).
(6) No-effect level/No-toxic-effect level/No-adverse-effect level/No-observed-effect level is the maximum dose used in a test which produces no observed adverse effects. A no-observed-effect level is expressed in terms of weight or volume of test substance given daily per unit volume of air (mg/l or ppm).
(7) Cumulative toxicity is the adverse effects of repeated doses occuring as a result of prolonged action on, or increased concentration of the administered test substance or its metabolites in susceptible tissues.
(c) Principle of the test method. Several groups of experimental animals are exposed daily for a defined period to the test substance in graduated concentrations, one concentration being used per group, for a period of 90 days. During the period of administration, the animals are observed daily to detect signs of toxicity. Animals which die during the test are necropsied and at the conclusion of the test, surviving animals are sacrificed and necropsied and appropriate histopathological examinations carried out.
(d) Test procedures -
(1) Animal selection -
(i) Species and strain. A mammalian species shall be used for testing. A variety of rodent species may be used, although the rat is the preferred species. Commonly used laboratory strains shall be employed. If another mammalian species is used, the tester shall provide justification/ reasoning for its selection.
(ii) Age. Young adult animals shall be used. At the commencement of the study the weight variation of animals shall not exceed ±20 percent of the mean weight for each sex.
(iii) Sex.
(A) Equal numbers of animals of each sex shall be used at each dose level.
(B) Females shall be nulliparous and nonpregnant.
(iv) Numbers.
(A) At least 20 rodents (10 females and 10 males) shall be used for each test group. If another mammalian species is selected (e.g. dog, rabbit, or non-human primate), at least 8 animals (4 males and 4 females) shall be used.
(B) If interim sacrifices are planned, the number of animals shall be increased by the number of animals scheduled to be sacrificed before the completion of the study.
(2) Control groups. A concurrent control group is required. This group shall be an untreated or sham-treated control group. Except for treatment with the test substance, animals in the control group shall be handled in a manner identical to the test group animals. Where a vehicle is used to help generate an appropriate concentration of the substance in the atmosphere, a vehicle control group shall be used. If the toxic properties of the vehicle are not known or cannot be made available, both untreated and vehicle control groups are required.
(3) Satellite group. A satellite group of 20 animals (10 animals per sex) may be treated with the high concentration level for 90 days and observed for reversibility, persistence, or delayed occurrence of toxic effects for a post-treatment period of appropriate length, normally not less than 28 days.
(4) Dose levels and dose selection.
(i) In subchronic toxicity tests, it is desirable to have a concentration-response relationship as well as a no-observed-toxic-effect level. Therefore, at least 3 concentration levels with a control and, where appropriate, a vehicle control (corresponding to the concentration of vehicle at the highest exposure level) shall be used. Concentrations should be spaced appropriately to produce test groups with a range of toxic effects. The data should be sufficient to produce a concentration-response curve.
(ii) The highest concentration should result in toxic effects but not produce an incidence of fatalities which would prevent a meaningful evaluation.
(iii) The lowest concentration should not produce any evidence of toxicity. Where there is a usable estimation of human exposure the lowest concentration should exceed this.
(iv) Ideally, the intermediate concentration level(s) should produce minimal observable toxic effects. If more than one intermediate concentration level is used, the concentrations should be spaced to produce a gradation of toxic effects.
(v) In the low and intermediate groups and in the controls the incidence of fatalities should be low, to permit a meaningful evaluation of the results.
(vi) In the case of potentially explosive test substances, care should be taken to avoid generating explosive concentrations.
(5) Exposure conditions. The animals should be exposed to the test substance, ideally for 6 hours per day on a 7-day per week basis, for a period of 90 days. However, based primarily on practical considerations, exposure on a 5-day-per-week basis for 6 hours per day is the minimum acceptable exposure period.
(6) Observation period.
(i) Duration of observation shall be for at least 90 days.
(ii) Animals in a satellite group scheduled for followup observations should be kept for at least 28 days further without treatment to detect recovery from, or persistence of, toxic effects.
(7) Inhalation exposure.
(i) The animals shall be tested in inhalation equipment designed to sustain a minimum dynamic air flow of 12 to 15 air changes per hour and ensure an adequate oxygen content of 19 percent and an evenly distributed exposure atmosphere. Where a chamber is used, its design should minimize crowding of the test animals and maximize their exposure to the test substance. This is best accomplished by individual caging. To ensure stability of a chamber atmosphere, the total “volume” of the test animals shall not exceed 5 percent of the volume of the test chamber. Oronasal or head-only exposure may be used if it is desirable to avoid concurrent exposure by the dermal or oral routes.
(ii) A dynamic inhalation system with a suitable flow control system shall be used. The rate of air flow shall be adjusted to ensure that conditions throughout the exposure chamber are essentially the same. Maintenance of slight negative pressure inside the chamber will prevent leakage of the test substance into surrounding areas.
(iii) The temperature at which the test is performed should be maintained at 22 °C (±2°). Ideally, the relative humidity should be maintained between 40 to 60 percent, but in certain instances (e.g., tests of aerosols, use of water vehicle) this may not be practicable.
(8) Physical measurements. Measurements or monitoring shall be made of the following:
(i) The rate of air flow shall be monitored continuously and recorded at least every 30 minutes.
(ii) The actual concentrations of the test substance shall be measured in the breathing zone. During the exposure period the actual concentrations of the test substance shall be held as constant as practicable, monitored continuously or intermittently depending on the method of analysis, and recorded at least at the beginning, at an intermediate time, and at the end of the exposure period.
(iii) During the development of the generating system, particle size analysis shall be performed to establish the stability of aerosol concentrations with respect to particle size. During exposure, analysis shall be conducted as often as necessary to determine the consistency of particle size distribution.
(iv) Temperature and humidity shall be monitored continuously but shall be recorded at least every 30 minutes.
(9) Feed and water during exposure period. Feed shall be withheld during exposure. Water may also be withheld during exposure.
(10) Observation of animals.
(i) Each animal shall be observed daily and, if necessary, handled to appraise its physical condition.
(ii) Additional observations should be made daily with appropriate actions taken to minimize loss of animals to the study (e.g., necropsy or refrigeration of those animals found dead and isolation or sacrifice of weak or moribund animals).
(iii) Signs of toxicity shall be recorded as they are observed including the time of onset, the degree, and duration.
(iv) Cage-side observations should include, but not be limited to, changes in the skin and fur, eyes and mucous membranes, respiratory, circulatory, autonomic and central nervous systems, somatomotor activity and behavior pattern.
(v) Animals shall be weighed weekly. Feed consumption shall also be determined weekly if abnormal body weight changes are observed.
(vi) At the end of the study period all survivors in the nonsatellite treatment groups shall be sacrificed. Moribund animals shall be removed and sacrificed when noticed.
(11) Clinical examinations.
(i) The following examinations shall be made on all animals of each sex in each group:
(A) Certain hematology determinations shall be carried out at least two times during the test period on all groups of animals including concurrent controls: After 30 days of test and just prior to terminal sacrifice at the end of the test period. Hematology determinations which are appropriate to all studies: Hematocrit, hemoglobin concentration, erythrocyte count, total and differential leukocyte count, and a measure of clotting potential such as clotting time, prothrombin time, thromboplastin time, or platelet count.
(B) Certain clinical biochemistry determinations on blood should be carried out at least two times during the test period on all groups of animals including concurrent controls: After 30 days of test and just prior to terminal sacrifice at the end of the test period. Clinical biochemistry test areas which are considered appropriate to all studies: Electrolyte balance, carbohydrate metabolism, and liver and kidney function. The selection of specific tests will be influenced by observations on the mode of action of the substance. Suggested determinations: calcium, phosphorus, chloride, sodium, potassium, fasting glucose (with period of fasting appropriate to the species), serum glutamic-pyruvic transaminase, (now known as serum alanine aminotransferase), serum glutamic-oxaloacetic transaminase (now known as serum aspartate aminotransferase), ornithine decarboxylase, gamma glutamyl transpeptidase, urea nitrogen, albumen, blood creatinine, total bilirubin, and total serum protein measurements. Other determinations which may be necessary for an adequate toxicological evaluation include: Analyses of lipids, hormones, acid/base balance, methemoglobin, and cholinesterase activity. Additional clinical biochemistry may be employed, where necessary, to extend the investigation of observed effects.
(ii) The following examinations shall be made on high dose and control groups. If changes in the eyes are detected, all animals shall be examined:
(A) Ophthalmological examination, using an ophthalmoscope or equivalent suitable equipment, shall be made prior to exposure to the test substance and at the termination of the study.
(B) Urinalysis is not recommended on a routine basis, but only when there is an indication based on expected and/or observed toxicity.
(12) Gross pathology.
(i) All animals shall be subjected to a full gross necropsy which includes examination of the external surface of the body, all orifices and the cranial, thoracic, and abdominal cavities and their contents.
(ii) At least the liver, kidneys, adrenals, brain, and gonads shall be weighed wet, as soon as possible after dissection to avoid drying. In addition, for the rodent, the brain; for the non-rodent, the thyroid with parathyroids also shall be weighed wet.
(iii) The following organs and tissues, or representative samples thereof, shall be preserved in a suitable medium for possible future histopathological examination: All gross lesions; lungs - which should be removed intact, weighed, and treated with a suitable fixative to ensure that lung structure is maintained (perfusion with the fixative is considered to be an effective procedure); nasopharyngeal tissues; brain - including sections of medulla/pons cerebellar cortex and cerebral cortex; pituitary; thyroid/parathyroid; thymus; trachea; heart; sternum with bone marrow; salivary glands; liver; spleen; kidneys; adrenals; pancreas; gonads; uterus; accessory genital organs (epididymis, prostate, and, if present, seminal vesicles); aorta; (skin); gall bladder (if present); esophagus; stomach; duodenum; jejunum; ileum; cecum; colon; rectum; urinary bladder; representative lymph node; (mammary gland); (thigh musculature); peripheral nerve; (eyes); (femur - including articular surface); (spinal cord at three levels - cervical, midthoracic, and lumbar); and (zymbal and exorbital lachrymal glands).
(13) Histopathology. The following histopathology shall be performed:
(i) Full histopathology on the respiratory tract and other organs and tissues, listed above, of all animals in the control and high dose groups.
(ii) All gross lesions in all animals.
(iii) Target organs in all animals.
(iv) The tissues mentioned in brackets (listed above) if indicated by signs of toxicity or target organ involvement.
(v) Lungs of animals (rodents) in the low and intermediate dose groups shall also be subjected to histopathological examination, primarily for evidence of infection since this provides a convenient assessment of the state of health of the animals.
(vi) When a satellite group is used, histopathology shall be performed on tissues and organs identified as showing effects in the treated groups.
(e) Data and reporting -
(1) Treatment of results.
(i) Data shall be summarized in tabular form, showing for each test group the number of animals at the start of the test, the number of animals showing lesions, the types of lesions, and the percentage of animals displaying each type of lesion.
(ii) All observed results, quantitative and incidental, should be evaluated by an appropriate statistical method. Any generally accepted statistical method may be used; the statistical methods should be selected during the design of the study.
(2) Evaluation of results. The findings of the subchronic inhalation toxicity study should be evaluated in conjunction with the findings of preceding studies and considered in terms of the observed toxic effects and the necropsy and histopathological findings. The evaluation will include the relationship between the concentration of the test substance and duration of exposure, and the presence or absence, the incidence and severity, of abnormalities, including behavioral and clinical abnormalities, gross lesions, identified target organs, body weight changes, effects on mortality and any other general or specific toxic effects. A properly conducted subchronic test should provide a satisfactory estimation of a no-effect level.
(3) Test report. In addition to the reporting requirements as specified under EPA Good Laboratory Practice Standards, 40 CFR part 792, subpart J, the following specific information shall be reported:
(i) Test conditions.
(A) Description of exposure apparatus, including design, type, dimensions, source of air, system for generating particulates and aerosols, method of conditioning air, treatment of exhaust air, and the method of housing animals in a test chamber.
(B) The equipment for measuring temperature, humidity, and particulate aerosol concentrations and size shall be described.
(ii) Exposure data. These shall be tabulated and presented with mean values and measure of variability (e.g., standard deviation) and shall include:
(A) Airflow rates through the inhalation equipment.
(B) Temperature and humidity of air.
(C) Nominal concentration (total amount of test substance fed into the inhalation equipment divided by volume of air).
(D) Actual concentration in test breathing zone.
(E) Particle size distribution (e.g., median aerodynamic diameter of particles with standard deviation from the mean).
(iii) Group animal data. Tabulation of toxic response data by species, strain, sex, and exposure level for:
(A) Number of animals dying.
(B) Number of animals showing signs of toxicity.
(C) Number of animals exposed.
(iv) Individual animal data.
(A) Date of death during the study or whether animals survived to termination.
(B) Date of observation of each abnormal sign and its subsequent course.
(C) Body weight data.
(D) Feed consumption data when collected.
(E) Hematological tests employed and all results.
(F) Clinical biochemistry tests employed and all results.
(G) Necropsy findings.
(H) Detailed description of all histopathological findings.
(I) Statistical treatment of results where appropriate.
(f) References. For additional background information on this test guideline the following references should be consulted:
(1) Cage, J.C. “Experimental Inhalation Toxicology,” Methods in Toxicology. Ed. G.E. Paget. (Philadelphia: F.A. Davis Co. 1970, pp. 258-277.
(2) Casarett, L.J., Doull, J. “Chapter 9.” Toxicology: The Basic Science of Poisons (New York: Macmillan Publishing Co. Inc. 1975).
(3) MacFarland, H.N. “Respiratory Toxicology,” Essays in Toxicology. Ed. W.J. Hayes. Vol. 7 (New York: Academic Press, 1976) pp. 121-154.
(4) National Academy of Sciences. “Principles and Procedures for Evaluating the Toxicity of Household Substances,” a report prepared by the Committee for the Revision of NAS Publication 1138, under the auspices of the Committee on Toxicology, National Research Council, National Academy of Sciences, Washington, DC (1977).
(5) World Health Organization. “Part I. Environmental Health Criteria 6,” Principles and Methods for Evaluating the Toxicity of Chemicals. (Geneva: World Health Organization, 1978).
[50 FR 39397, Sept. 27, 1985, as amended at 52 FR 19073, May 20, 1987; 52 FR 26150, July 13, 1987; 53 FR 49150, Dec. 6, 1988; 54 FR 21064, May 16, 1989]