The smoothing effect of ventilation on the variations of air contaminant concentration is calculated. An additional smoothing effect is produced by the instruments employed to measure the concentration. The time constants of the body reacting to an inhaled contaminant produce a similar and more marked smoothing effect on the body burden. The way air sampling results may be interpreted in terms of the consequent effect on body burden is worked out. Appropriate allowances for extraordinary work schedules are given and the uses of personal sampling and fixed position sampling are discussed.