INTRODUCTION A hazard assessment for gas is a comprehensive description of the nature of gas in the ground at a particular site, and how it may move into a tunnel or shaft. The assessment is important to design of atmosphere control measures for con­ struction. It should be performed prior to excavation, and should be based on the nature of gas in the ground as ascertained from quantitative data. Assessments based mainly on indications of gas in previous excavations in the area, or in an advancing excavation, are prone to error. An inaccurate assessment may delay installation of appropriate atmosphere control measures, and raise the risk of a serious accident in the interim.