In any release of a chemical or biological agent, the nature and degree of hazard will depend on a multitude of factors, including, among other things, the agent and the amount released, the method by which the agent is disseminated, factors that influence its toxicity, infectivity or virulence both during and after its release, its movement and dilution in the atmosphere, and the state of protection and susceptibility of those exposed. Two different types of general hazard are usually distinguished, namely inhalation hazard and contact hazard, with different characteristic implications for protection.
For infective agents, the principal hazard to people will be from inhalation. For many infective agents, the hazard is greatest if the agent reaches the target population in the form of particles within a narrow size range, i.e. small enough to penetrate to the alveoli in the depths of the lungs but not so small that most of the particles fail to be deposited and instead are mostly exhaled. Thus the optimum size for particle is generally thought to be somewhere between 0.5 and 5 microns. Contact with an infective agent and its entry into the body via a wound or via mucous membranes may also present a hazard, although generally much less than that from inhalation. Infective agents may be disseminated as respirable particles by explosives or by sprayers or other generators specially designed to produce particles in the respirable size range.
For chemical agents, an inhalation hazard may be created by the dissemination of the agent as a vapour, as liquid or solid particles sufficiently small to be respirable, as a spray that evaporates to form a vapour while still airborne, or as a spill or spray that is deposited on surfaces and subsequently evaporates to form a vapour. For some agents, vapours or respirable particles may also present a hazard to sensitive mucous membranes, especially those of the conjunctiva. For chemical agents able to act percutaneously (effected or through the skin), a contact hazard may be created by sprays or spills of less volatile agents deposited directly on people or on surfaces with which people are likely to come into contact. A chemical agent may be disseminated mechanically by spraying or rupturing a container, by using explosives, or by a thermal process in which a pyrotechnic composition is used as the source of heat. Pyrotechnic dissemination is effective only for heat-resistant and non-combustible agents, which may evaporate initially and then condense as a suspension in air of respirable particles, creating principally, an inhalation or conjunctive hazard.