Oxygen deficiency and toxic gas: sending the drone in before the person

Normal air contains about 21% oxygen. In enclosed spaces, rusting, microbial decomposition, fermentation and inert-gas purging consume or displace oxygen without anyone noticing. Japanese regulations define below 18% oxygen as oxygen deficiency, and hydrogen sulphide above 10 ppm is also regulated. The other danger is toxic gas: hydrogen sulphide from sewage and sludge, carbon monoxide from combustion and exhaust. Both can incapacitate a person in small amounts.

You meet oxygen deficiency and toxic gas in sewers and manholes, utility tunnels, inside storage tanks and in mines. All have narrow entries, poor ventilation and dark interiors, so the walls of confined spaces, darkness and GNSS denial usually arrive together.

• Send the drone in ahead of people: check conditions and gas readings before anyone enters. This also finds spaces nobody needs to enter at all.
• Measure at several heights: stop and measure near the floor, at mid-height and near the roof. H₂S and CO₂ pool low; methane collects high.
• Combine with ventilation: measure, ventilate, measure again — and expect conditions to worsen once ventilation stops.
• Prepare for corrosion: in H₂S-rich spaces boards and connectors degrade. Build post-flight inspection and cleaning into the procedure.
• Decide rescue procedures first: if the drone falls inside, nobody goes in after it. Recover it only after re-ventilating and re-testing, by a set procedure.

In Japan, the Ordinance on Prevention of Anoxia (酸欠則) governs work in places at risk of oxygen deficiency, such as sewers and tanks (which places qualify is set by a schedule to the Industrial Safety and Health Act’s enforcement order). It requires measuring oxygen (and in some places hydrogen sulphide) before work, ventilation, appointment of a work chief and protective equipment. Drone measurements help reveal danger before human measurement, but how the legally required measurements are satisfied should be decided against the regulations and the views of the competent authority.

Abroad, for example, US OSHA regulates permit-required confined spaces, requiring atmospheric testing before entry and an attendant outside. Related: explosive atmospheres, confined spaces, world regulations.

• Trust in airborne readings: response lag, downwash, temperature and humidity mean airborne sensors cannot be assumed as accurate as handheld detectors on the ground.
• Deep inside long pipes: radio and endurance run out deep in sewers; at many sites only the first few tens of metres from the entry can be measured.
• Visualising gas: leak-finding outdoors is advancing, but mapping concentration inside enclosed spaces is largely still research.
• Corrosion and fouling: H₂S and sludge spray shorten aircraft and sensor life.

When a manhole opens, a small aircraft descends first and sends gas readings by height and a 3D view of the interior to the work chief’s tablet. The readings are automatically cross-checked against fixed-sensor records, with a suggestion like “ventilate for N minutes today.” People enter only when truly needed, and secondary casualties among would-be rescuers have become rare. That is the editors’ outlook.