"The Deadly Drain:" Probable hydrogen sulfide poisoning, Unaizah, Al-Qassim region, May 28, 1998

On June 8, 1998, the Field Epidemiology Training Program (FETP) was notified by the King Saud Hospital in Unaizah of two sudden deaths occurring on May 28, 1998, which apparently were caused by exposure to an unidentified chemical in the bathroom of a compound housing 41 furniture workers.

On-site investigation of this incident of probable hydrogen sulfide (H2S) poisoning began immediately by members of FETP. Surviving workers and involved medical staff at the hospital were interviewed to establish the time sequence and description of the fatal accident. Questionnaires were distributed to all compound residents covering demographic data, treatment of bathrooms, which chemicals were used, observations about the accident site, and medical problems experienced as a result of the accident.

The workers reported that 30 minutes after pouring a solution of 98% H2SO4 into 7 of the 8 drains in 8 bathroom stalls, one of the workers returned to bathe in the only untreated stall (Stall #8). After 45 minutes he was found by a co-worker lying immobile on the floor of the stall. This friend also collapsed inside the stall. Two other rescuers collapsed immediately outside the stall. At the hospital 20 minutes later, the first two workers were pronounced dead. The other two recovered completely within 24 hours using only oxygen. No physical or physiologic abnormalities were detected on the two surviving workers at the hospital.

We measured, mapped, and inspected the toilets, drains, and sewage system of the compound. We generated smoke in the sewage line to retrace the pathway followed by gasses generated in the sewage. All eight stalls were 2m3 with a tight fitting door rising 1/2cm above the floor and 1/2m from the ceiling. Drains from the eight stalls joined into a common drain line, then to a 32m holding tank 3m down line. This system had no vertical venting pipe. Smoke generated in the line near the tank appeared in Stall #8 after less than 1 minute and rapidly rose to lm high inside the stall (Figure 1). No smoke appeared in any other stall nor from the lid on the sewage tank.

The drains on all the stalls were replaced and the system was equipped with a vertical ventilation pipe.

Hydrogen sulfide (H2S) is a colorless, heavier than air gas with a toxicity comparable to that of cyanide [1]. Hydrogen sulfide can be produced naturally in sewage from the soluble sulfide and hydrosulfide salts and sulfur-containing peptides (such as keratin) which are converted to H2S by certain strains of bacteria. A common bacteria that produces H2S from protein is Proteus vulgaris. Two genera of anaerobic bacteria, Desulfovibrio (5 species) and Desulfotomaculum (3 species), can reduce sulfur compounds (e.g. sulfate) to H2S [2].

Breathing H2S at concentrations greater than 500ppm of air can be fatal in just a few breaths, causing respiratory arrest. Death is usually preceded by a loss of consciousness after one or more breaths, although loss of consciousness does not necessarily mean that death will follow [3]. Although the hospital medical findings were not conclusive for H2S, the multiple findings of this investigation, including rapid knockdown and death; the rotten egg smell; eye and throat irritation; and headache reported by survivors, is highly characteristic of H2S. The temporal association with sewer cleaning also strongly suggests H2S, which can be released rapidly and unpredictably from

sewage Finally, the smoke generation test showed that any gas generated in the sewage lines would rapidly appear in Stall #8 and would concentrate in that stall because of the tight fitting door. All of these findings suggest that this incident was due to H2S poisoning.

We believe that H2S generated when H2SO4 was added to the sewage system. When the first victim bent over to fill his bucket with water he probably inhaled, filling his lungs with a toxic concentration, and then fell to the floor where he continued breathing a high concentration until he died. When the second worker attempted a rescue, the stall had much more time to accumulate H2S and he was overcome immediately, as witnessed.

Using a solution of 98% H2SO4 to clean drains and sewage lines is a common practice in Saudi Arabia, and many sewage disposal systems probably have similar designs. We recommend that warnings about using acid drain cleaners in closed sewage systems be circulated to municipal authorities, employers of laborers, and to the general public.