Catálogo de publicaciones - libros

The external environment at 41000 ft (12500 m), a typical cruise altitude for modern civil aircraft, is hostile to human life. Aircraft environmental control systems are designed to ensure the survival of the aircraft occupants as well as providing them with a comfortable atmosphere. Major design drivers for the environmental control system are thermal comfort, pressurisation and cabin air quality. However, these parameters cannot be considered independently. They interact between themselves and with other parameters, which may or may not be controllable by the system designer. These interactions occur in a highly complex manner. Research has led to a good understanding of the basic functions to allow safe and comfortable aircraft environmental conditions. Future research efforts will be increasingly focussed on identifying and elaborating the interdependency of factors in order to further enhance the aircraft cabin environment.

Symptoms reported by passengers and crewmembers on commercial aircraft are described according to individual air quality-related sources, including: (1) elevated levels of bioeffluents; (2) infectious agents; (3) extreme temperatures; (4) exhaust fumes, deicing fluid, fuel fumes, and cleaning products; (5) heated engine oil and hydraulic fluid; (6) reduced oxygen supply; (7) ozone gas; and (8) insecticides. A brief overview of the aircraft regulatory environment and available sources of data on the hazards and associated health effects is also provided.

The purpose of heating or cooling systems is to provide an acceptable microclimate and maintain suitable conditions for the intended use of the space. Airliner cabins, however, present different design and operation challenges because of the extreme external environmental conditions, the complexity of the operational systems and the diverse authorities that govern such environments. The environmental quality of a space is determined by the occupant's response to various environmental stimuli and his integration of these inputs into a comfort and health response. Most thermal comfort studies have dealt with a homogeneous population with similar physical characteristics (neither ill nor old). The concept of comfort to meet the requirements of the elderly and the new health metric called the disability-adjusted life year (DALY) are introduced. This contribution reviews environmental requirements such as ventilation, relative humidity, carbon dioxide concentrations, ozone and pressure as expressed in relevant guidelines and standards that are applicable to air quality in air cabins. Further, the health effects associated with environmental exposures organized by level of concern are discussed. Based on existing data, it is concluded that at cruise altitude the pollutant of primary concern is ozone (O). Ozone standards are not regularly met. Carbon monoxide and particulate matter concentrations appear to be lower than health-based standards for ambient air, while VOC and SVOC appear to be present in similar concentrations as in other transportation vehicles.

Atmospheric pressure is reduced as a function of altitude, thus making hypoxia, the condition of oxygen deficiency, a concern for aviation. The effects of low grade hypoxia are often subtle and may be missed by both flight crews and passengers. The most severe effects are widely appreciated when high profile incidents occur. The international collective was stirred after October 25, 1999 when Payne Stewart, a professional golfer, and five other persons were killed when a Learjet flew on unmonitored autopilot for approximately four hours before running out of fuel and crashing in South Dakota. Radio contact was lost after the aircrew acknowledged clearance to an altitude of 11900 m (39000 ft). The accident investigation determined that the crew was incapacitated when inadequate supplemental oxygen delivery followed a loss of cabin pressurization. This chapter will discuss the physics and physiology of hypoxia, describe cabin pressurization and discuss the health effects at normal cabin pressure and following unplanned depressurization.

A deep vein thrombosis (DVT) is essentially the formation of a clot in the veins of the leg. This causes obstruction to the normal flow of blood in the limb which can result in pain and swelling of the leg. Occasionally, a fragment of the clot can break off and pass in the blood stream to the heart or major blood vessels leading into the lungs from the heart. This phenomenon is known as pulmonary embolism and has been estimated to occur in approximately 1% of cases of deep vein thrombosis. The long-term consequences of venous thromboembolism are not insignificant and include risk of recurrence and post-phlebitic syndrome. It is now generally accepted that there is an association between any form of long-distance travel and venous thromboembolism and therefore the alternative term of “travellers' thrombosis” has been suggested as an alternative to the term “economy class syndrome.” Thromboembolism is rarely observed after flights of less than 5 h duration and, typically, the flights are of 12 h or more. Stasis in the venous circulation of the lower limbs is undoubtedly the major factor in promoting the development of venous thromboembolism associated with travel. Some individuals may be particularly predisposed to develop venous thrombosis because of congenital (inherited) deficiencies of natural anticoagulants, such as antithrombin, protein C or protein S. However, routine screening of passengers for these abnormalities is not justified or cost effective but may be of value in selected cases. It has also recently been suggested that exposure to mild hypobaric hypoxia in pressurized aircraft may also result in activation of the coagulation cascade but the data are conflicting. The risk of venous thromboembolism is largely confined to those with recognized additional risk factors for venous thromboembolism. Leg exercises whilst seated help to reduce the risk of DVT. There is also clear evidence from prospective and randomized clinical trials to support the use of compression hosiery as a preventative measure. By contrast, there is no firm evidence to support the indiscriminate use of aspirin as a routine prophylactic measure. Airlines have recently taken positive steps to address the issue of air travel and thrombosis. At the same time, the travelling public needs to be more aware of the issues and assume some responsibility for ensuring fitness to fly and the choice (and therefore cost) of their seats.

The incidence of disruptive behaviour by passengers in civil aircraft is unknown due to under-reporting. The theoretical pathophysiology is discussed. Underlying aggressive personality and life stress predispose to disruptive behaviour. The use and misuse of certain drugs including alcohol further sensitise susceptible individuals who are then “triggered” by conditions induced by the aircraft cabin environment, especially mild hypoxia and nicotine withdrawal.

Air travel is associated with crowded conditions that can facilitate the transmission of airborne infectious diseases. The risk of contracting such diseases depends on the presence of an infected person who is shedding infectious particles and sufficient exposure of a sensitive person to achieve an adequate dose to cause disease. Proximity to the infectious person and the length of time spent near the person are the most important risks for contracting a disease. Ventilation patterns play a lesser role in disease transmission. Well-documented outbreaks of influenza, severe acute respiratory syndrome (SARS), and tuberculosis have occurred. Other common respiratory illnesses have probably also been spread via aircraft, but outbreaks remain unrecognized. Research on the spread of infectious disease in aircraft has focused on sampling for microorganisms in air (which has little relevance), and on the development of models to predict the risks for specific diseases.

Microorganisms that affect human health are found in all indoor environments, including cabins of commercial aircraft. Those that arise from human sources can be transmitted by direct contact, droplets, or the airborne route. Infections from human sources include Influenza, Rhinovirus, SARS and tuberculosis. Transmission by the airborne route can be reduced by sterilizing the air with ultraviolet germicidal irradiation, or by diluting the contaminated air with outdoor air through ventilation. Microbes arising from environmental sources include bacteria, fungi and other organisms such as protozoa. These usually have very simple requirements for growth – water and a simple substrate such as dust. They cause health effects through direct infection rarely (one example is Legionnella), but more commonly cause immune reactions resulting in hypersensitivity or allergy mediated diseases. Environmental sources of microbial contamination are best prevented, but can be remediated through cleaning, germicidal chemicals, or ultraviolet germicidal irradiation. Airborne microbial substances including toxins, antigens and viable organisms can be removed by outdoor air ventilation or filtration. In aircraft cabins transmission of pathogens from human sources is difficult to control, but airborne transmission can be reduced through increased outdoor air ventilation or filtration. Environmental microbial contamination can, and does occur in aircraft cabins. These microbial sources are best prevented but, if detected, can be removed through cleaning or disinfection. Ultraviolet germicidal irradiation is an under-utilized technology that may be useful for sterilizing air as well as potential environmental sources.

Insecticides are applied in the aircraft cabin for four key reasons: (1) to comply with foreign quarantine regulations applicable to certain international flights; (2) to control insects in the aircraft galleys where food and food waste are stored; (3) to respond to insect sightings reported by passengers or crew; and (4) to combat seasonal insect populations. Insecticide application related to foreign quarantine regulations has generated the most controversy and concern for crew and passenger health. Forty seven countries require that the cabin and cockpit of commercial aircraft are sprayed with insecticides, either prior to or upon arrival, to protect against importing insects that may be on board and may carry disease or damage the environment. Spraying practices vary widely between countries and airlines. Although the World Health Organization describes these practices as safe “if carried out with the recommended precautions,” little or no attention is paid to exposure control practices. Government agencies, labor unions, airlines, and environmental groups have received reports of ill health from passengers and crew, with symptoms that range from rash to anaphylaxis. The current focus is on developing mechanical methods of disinsection that will satisfy countries' quarantine concerns without compromising the health of aircraft occupants.

Although air quality incidents in aircraft occur at low frequencies, ranging from 1 per 10000 flights to 3.8 per 1000 flights depending on aircraft type, these are not rare events considering there are close to 30000 flights per day in the USA alone. An analysis of the reports by pilots and flight attendants indicates that the majority of reported symptoms fall into the category of central nervous system impairment, followed by problems with the respiratory system. In addition, the majority of mechanical problems that were identified as the cause of these incidents were associated with oil contamination of the air compressor stages of the engine and the auxiliary power unit (APU). In addition, in some aircraft types, hydraulic fluid contamination of the APU air intake was also frequently reported. Analysis of jet engine lubrication oils and hydraulic fluids indicates these agents can be a source of carbon monoxide and tricresyl phosphates. Exposure to either of these agents has been linked to central nervous system impairment. Identification of contaminants released into the air during such incidents is virtually non-existent as it would require a large number of air quality monitors to be placed on aircraft in order to capture these rare events. As a solution to this problem a small inexpensive air sampler has been developed that is self-contained and can be activated by anyone. This sampler also has a direct-reading CO monitor that can be used to provide an objective criterion for triggering the air sampler during an event. The exposed sampler can then be forwarded to the laboratory for analysis of oil contaminants using gas chromatography--mass spectrometry (GC-MS). In this fashion a data base can be accumulated that provides an objective measure of exposures during these incidents and whether these exposures can be linked to the symptoms that have been reported by flight crew personnel. A GC-MS analysis has the additional benefit of identifying potential synergistic agents, such as the pesticides used to disinsect aircraft.