In the absence of any data on the p

In the absence of any data on the probability of occurrence of harm, it is not possible to reach any riskestimate, and it is usually necessary to evaluate the risk on the basis of the nature of the harm alone. If it canbe concluded that the hazard is of little practical consequence, the risk can be judged to be acceptable and norisk control measures are necessary. However, for significant hazards, that is, hazards which could inflictharm of high severity such as those noted above, no level of exposure can be identified that corresponds to arisk so low that there is no need to bother about it. In such cases, the risk estimate should be made on thebasis of a reasonable worst-case estimate of probability. In some instances, it is convenient to set this defaultvalue of the probability to one and to base risk control measures on preventing the hazard entirely, reducingthe probability of harm to an acceptable level or in reducing the severity of the harm (see D.4).