Everyone is exposed to a great variety of chemical substances in the normal course of their life, both at work and away from work. Most of these substances do not present a hazard under normal circumstances, but have the potential for being injurious at some sufficiently high concentration and level of Exposure. Toxic substances, or Poisons, are those materials which can have an injurious effect when introduced into, or absorbed by, a living organism. In this same context, Corrosive materials are included because they may damage or destroy living tissues. The terms Acute and Chronic are used frequently in connection with both toxic exposure and toxic effects. Although used in their normal sense, care is necessary in the use of these terms. Acute implies short duration, while chronic implies a prolonged or recurrent nature. Short-term accidental exposures would therefore be termed acute while daily exposures to background concentrations in the workplace would be termed chronic. To avoid confusion, it is recommended that the terms acute and chronic should not be used to infer high and low concentrations of toxic materials. Their meaning should be restricted to describe time rather than severity, in line with the medical profession in their description of disease. Some effects do not arise immediately after exposure and are termed delayed or latent. These terms are obviously relative and confusion can arise between exposure to a Carcinogen where the induction period before the appearance of harm (if it does arise) may be many years and exposure to toxic agents where a person may appear to survive a lethal exposure only to die after a day or two. For some substances, a very small quantity may cause considerable harm whereas for others, a much larger quantity may be required to cause a harmful effect. It is this relative power of a toxic material to cause harm that is termed Toxicity. It is important to distinguish toxic and corrosive materials from those, which are purely narcotic or Irritant. The latter may cause pain and discomfort through immediate or prolonged contact with the skin, but they do not themselves harm or destroy living tissue. Similarly, narcotic substances dull the senses and impair reactions without necessarily causing permanent damage. Materials, which are not toxic, irritant or corrosive, may still endanger life if present in high enough concentrations in the atmosphere by reducing the oxygen content and thus may lead to Asphyxiation. Two types of occupational exposure limit are now in use; Control Limits, which should not normally be exceeded, and Recommended Limits, which are considered to represent good practice. For each type of limit, two types of exposure are considered. The Long Term Exposure Limit is concerned with reducing the risk from total intake, day after day, over long periods. The Short Term Exposure Limit is aimed primarily at avoiding acute effects from brief exposures or peaks in exposure. Both long and short term exposure limits are expressed as time weighted average concentrations, the long term exposure limit is normally averaged over eight hour periods and the short term exposure limit over a ten minute period. Some of the short term exposure limits were formerly expressed as ceiling values which should not be exceeded even instantaneously, but this could not be monitored in practice since all samples need to be taken over a finite period. Ten minutes is considered to be the shortest practical time over which most personal samples can be taken at the levels of the exposure limits due to the limitations of available sampling and analytical techniques. Presently, control limits are only specified for a small number of hazardous substances. It should be noted that further action to reduce exposure below the limits may be necessary to fulfil legal requirements, particularly in the case of substances for which there is no apparent threshold below which adverse effects do not occur. The limits should not be used as an index of relative hazard to toxicity and do not represent a sharp dividing line between safe and dangerous concentrations. Until recently the term Threshold Limit Value (TLV) has been used to describe exposure limits. TLV's have traditionally been quoted in three forms. The Time Weighted Average (TLV-TWA) has been used to cover long term effects, the Short Term Exposure Limit (TLV-STEL) has been used to cover acute effects from a few minutes exposure, and the term Ceiling (TLV-C) has been used to cover situations where even the briefest of exposure is likely to cause harm. Many attempts have been made to categorise toxic materials according to the health hazards that may result from single high-level exposures. The majority of these classifications are based on the Lethal Dose administered, or Lethal Concentration inhaled over a period of 4 hours, that results in the death of 50 per cent of a test group within 14 days. This is the concept of the terms LD50 and LC50 respectively. Percentage other than 50 may be quoted. It should be noted that the term LC50 is often used loosely for exposure durations of other than four hours in which case the exposure period should be stated. A further concept in categorising actual health hazards is that of conditions which are Immediately Dangerous To Life or Health (IDLH). This term originated in the USA and was developed with reference to escape from highly toxic atmospheres. It can be confusing since the definition also refers to conditions, which could have cumulative or delayed effects on health