How many heaters can be activated at one time if we are in a confined space such as in the cabin of a truck (6' x 6') or in a mine shaft?

The main consideration is not the size of the space but ventilation. A minute amount of hydrogen is given off. This displaces oxygen. That is why we say "Activate the heater in a well-ventilated area". There are, however, no toxic fumes.

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