Summary

Oxygen self-rescuers are the most effective respiratory equipments for escape in an irrespirable area caused by a fire, explosion or gas outburst in underground mines. The apparatuses are classified as closed-circuit breathing apparatus because the user's exhaled air is rebreathed after exhaled carbon dioxide is removed by chemical agent and the oxygen content is augmented from stored supply in compressed or chemical form. These necessary components impose some physiological stressors, including above ambient levels of carbon dioxide (CO₂), high inhalation temperature and high breathing resistance. For this reason, oxygen self-rescuers are required to pass the certification tests for usage in Japanese underground mines. But the criteria for evaluation of the performance were not appropriate because of the lack of physiological data related to the apparatus, and there were following problems in the evaluation.

1. The heat stress of the inhaled air was evaluated according to the dry-bulb temperature without any consideration of humidity. It has been demonstrated that the heat perception is dependent on humidity as well as the dry-bulb temperature, so we did not evaluate the thermal impact of the inhaled gas appropriately.
2. The effects of hyperoxia and hypercapnia on metabolism have not yet been well defined, so there were possibility of misunderstandings about the necessary oxygen supply and carbon dioxide absorbent capacity.
3. One of the most serious effects of the elevated CO₂ on human body is the increase in ventilation rate, but we did not have enough information about the effect during exercise. Considering the apparatuses tend to be used under working condition, the permissible level of inhaled CO₂ might not be correctly determined.
4. Breathing resistance should be evaluated as the combined effects with increased carbon dioxide, because CO₂ stimulates breathing and extends the pressure fluctuation. But the luck of basic information about the combined effects might make the permissible level of breathing resistance unsuitable. 
5. Since oxygen self-rescuers are used in an emergency situation, the users would run as fast as possible and the work intensity would be very high. The present certification test only evaluated the performance of the apparatuses under light work condition. So using the apparatuses under 

high work intensity, the possibility of occurring unpredictable accidents would be increased.

The purpose of this study is to clarify the effects of the physiological stressors related to oxygen self-rescuers on human body and make some proposals to improve the certification tests and standards for the apparatuses. 

The details of chapters each are follows;

Chapter 1 is concerned with introduction as mentioned above.

Chapter 2 is concerned with the characteristics of oxygen self-rescuers, the present certification tests and standards, and a brief review of the physiological studies related to the apparatuses.

Chapter 3 is concerned with elevated breathing gas temperature and humidity. Elevated inhalation temperature is one of the most serious physiological stressors, and is caused by a chemical reaction removing the exhaled carbon dioxide or generating oxygen to compensate for the oxygen consumption of users. We needed enough information about the effects of inhaled temperature and humidity to confirm the appropriate criteria to evaluate the heat stress of the apparatuses. Two types of test were conducted, and it has been demonstrated that, at a moderate exercise level, the wet-bulb temperature was the controlling factor in the user's perception of heat, the dry-bulb temperature had no effect on the heat perception, and the maximum breathable temperature did not practically depend on the ventilation rate. And the wet-bulb temperature of the apparatuses were measured and it has been proved that the inhaled gas of KO₂ type is dry while the gas of compressed oxygen type is saturated with water vapor. We propose the introduction of wet-bulb temperature as an indicator of the heat stress in the certification tests, and 48°C as the maximum permissible wet-bulb temperature of inhaled air for the apparatuses.

Chapter 4 is concerned with the effects of change in inhaled gas content on human body. Elevated CO₂ concentration is one of the most important physiological stressors with oxygen self-rescuers because of the increase in ventilation rate due to the CO₂ stimulus. In order to ensure the safety performance of the apparatuses at moderate to high work rates, the ventilatory response to CO₂ during exercise is important in deciding the appropriate permissible level of inhaled CO₂. Thirteen healthy subjects volunteered the tests, and it has been demonstrated that the ventilatory response of the most sensitive subject was more than ten times that of the least sensitive subject, and the slopes of the ventilation and end-tidal CO₂ relationship were about 70% greater at moderate exercise than at rest. 

The effects of high oxygen and CO₂ concentration on respiration and metabolism are also important, because the breathing circuits of oxygen self-rescuers are closed system. Most of the fundamental physiological data used for designing the apparatuses were obtained using human subjects breathing ambient air at field tests. To confirm the effects more preciously thirteen healthy subjects performed bicycle-ergometer cycling at approximately 50% of their maximum working capacity while inhaling four different gas compositions: 1) Normoxia and Normocapnia (room air); 2) Hyperoxia (40%O₂); 3) Hypercapnia (3%CO₂); 4) Hyperoxia and Hypercapnia (40%O₂ and 3%CO₂). Hyperoxia decreased the ventilation rate about 7% with normocapnia and about 6% with hypercapnia. It also decreased the carbon dioxide output rate about 5% under both normocapnia and hypercapnia. Hyperoxia had no significant effect on the oxygen uptake rate. Hypercapnia increased the ventilation rate approximately 40% compared to normocapnia, but it did not change the rates of oxygen uptake or carbon dioxide output significantly. The effect on ventilation rate is very important factor for consideration of the safety of the apparatuses, since the increase in ventilation rate enlarges the pressure fluctuations in the respiratory circuit with breathing resistances and causes a physiological stress. So we want to recommend that the permissible inhaled CO₂ limit should be lowered from 3% to 2%. The sizing of oxygen supply of an apparatus at its expected work load can rely on data acquired while breathing air at ambient rather than at conditions simulating oxygen self-rescuer (hyperoxic and hypercapnic) which would make data acquisition more difficult. And sizing the CO₂ absorbent capacity for the apparatuses can also rely on physiological data acquired while breathing ambient air at field tests because a decrease in the CO₂ output rate with hyperoxia works on the safe side by prolonging the life time of the absorbent.

Chapter 5 is concerned with the effects of breathing resistance on human body. Firstly we decided the end-tidal CO₂ concentration as an indicator describing the intensity of physiological stress caused by breathing resistance. And secondly we determined the effect of breathing resistance with elevated inhaled CO₂ concentration by which ventilation rates and pressure fluctuations are increased. Eight male subjects volunteered for the study and breathed both normal air and 3% carbon dioxide with both low and high breathing resistance during moderate steady-state exercise. The high breathing resistance produced a pressure level that was almost the same as the upper permitted limit for the apparatuses in the Japanese standard. Increasing the breathing resistance did not cause any significant effect on the human body while breathing normal air but, in the case of 3% carbon dioxide, the high resistance increased end-tidal CO₂ significantly and caused strong distress in the test subjects. Based on these results, it is recommended that the permissible breathing resistance should be lowered to ±500Pa evaluated with a lung simulator breathing 30L/min.

Chapter 6 is concerned with the evaluation of safety of oxygen self-rescuers under heavy work conditions. Three units, two chemical-oxygen types and one bottle-oxygen type, were evaluated by the lung simulator and a human subject. The metabolic rate in the simulator tests was equivalent to light work; the human-subject tests were at a heavy work load. It was found that some apparatuses experienced a rapid increase in inhaled CO₂ when the metabolic rate of the user exceeded the absorbent capacity of the units. In those cases, the user couldn't continue walking nor breathing through the unit. To ensure the safety of the user, we propose that the certification standards would include the test of CO₂ absorbent capacity under heavy work conditions.

Chapter 7 is concerned with the conclusions of the study.

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