It is pointed out in the passage that one of the safety measures taken in an explosives factory is ----.

A typical explosives factory is divided into two parts:
the "non-danger" and "danger" areas. The main
business of the non-danger area lies in the
manufacture of nitric and sulphuric acids for the
nitration processes, including the recovery of these
acids from the waste products of nitration. Other raw
materials are also prepared in the non-danger area.
The actual manufacture of explosives and their mixing
and packing are carried out in the danger area, subject
to rigorous safety measures. The main danger in
manufacture is ignition by spark, friction or impact, the
latter two being especially hazardous if the explosive
is allowed to become contaminated with gritty material.
Naked lights, steel tools or anything which might
produce spark or flame are therefore excluded from
the danger buildings. Each building has a "clean" floor
which may be approached only in specially cleaned
shoes, while the workers are provided with factory
clothing to ensure that grit is not carried into the
buildings.