The hazards of hydrogen as used for the detection of shunts has been studied under clinical conditions with the Standard Mine Safety Appliance Explosimeter and under additional simulated circumstances far more extreme than would ever be encountered in a catheterization laboratory. Because of the lightness of the gas and its ready diffusibility, explosive concentrations were transiently identified in small zones that were grossly different from those reportedly associated with “heavier than air” explosive gases. With its use in single inhalations for the identification of an intracardiac shunt and with allowances for a liberal safety factor, no explosive risk exists outside a conical envelope, which may be considered to extend 3 feet above, 6 inches below, and 12 inches laterally in all directions from the patient's mouth. Ordinary electronic apparatus, motor-driven syringes, and x-ray equipment, provided they are commonly grounded and outside this zone, require no explosive safety precautions in their use when hydrogen is used in this manner for shunt detection. Room ventilation is desirable for personal comfort but should not be considered a safety requirement. Release of large quantities of hydrogen has shown that there is no reason to specify a time interval between inhalations for the dispersion of hydrogen.These conclusions are not intended to bemean those measures that are routinely followed to avoid the generation of static electricity.16But were a spark generated, it would be of no significance unless it occurred in the restricted zone described.