The Accident

May 11, 1976, was a beautiful spring Tuesday in Houston, Texas. Temperatures neared 80 degrees Fahrenheit (26 degrees Celsius), humidity was low for Houston (around 79 percent), and a nice (5.8 mph; 9.3 kph) breeze blew from the west-southwest.

As lunchtime neared, just after 11 AM, a tanker truck rounded an overpass ramp connecting I-610 with I-59. A combination of excessive speed and surge in the partially filled tank caused the driver to lose control and crash into the barrier. The barrier did not hold. The truck fell some 15 feet onto I-59 below. Ammonia cloud five minutes after the accident

Instantly upon impact the 7,509-gallon (about 19-ton) load of anhydrous ammonia was released. Within seconds, people at the intersection gasped for breath as they were enveloped by the overpowering odor of the highly toxic cloud of ammonia gas. Six people died; five of those died from inhaling ammonia. Another 178 sought medical attention; 78 of those were hospitalized with serious injuries.

This occurred less than two miles from the upscale Galleria retail outlet and about only 8.5 miles from downtown Houston.

 

The Science

Ammonia is colorless and lighter than air. It is soluble in water, highly corrosive, and extremely irritating to the skin, eyes, nose, and respiratory tract. Anhydrous ammonia is used commercially. The term anhydrous means without water. Because ammonia (NH3) boils at -28 degree Fahrenheit (-33.34 degree Celsius), the liquid form (which the truck carried) must be stored under high pressure or at low temperature.

The permissible exposure limit (PEL) of ammonia is 35 parts per million (ppm) for short-term exposure; 300 ppm is deemed immediately dangerous to life or health (IDLH).

Affects of ammonia exposure range from irritation to death:

  • 400–700 ppm: Severe eye and respiratory irritation can occur and potentially permanent damage
  • 1700 ppm: Convulsive coughing and bronchial spasms occur; 30 minutes of exposure at this level can be fatal
  • 3000 ppm: Intolerable and life threatening (severe respiratory tract damage, bronchitis, chemical inflammation of the lungs, fluid in the lungs, death)
  • 5000–10,000 ppm: Death can occur from suffocation
   

In this accident, a 10,000 ppm ammonia cloud stretched more than a half mile—1968 feet (600 meters) long and 1148 feet (350 meters) wide—the red oval on the dispersion map; 1200 ppm could be measured almost three-quarters of a mile away—3707 feet (1,130 meters) long and 1312 feet (400 meters) wide—the blue oval on the dispersion map.

All victims were within 1000 feet (304 meters) of the release site and were outside of their vehicles, either trying to leave the area or to help others; fatalities and serious injuries occurred within about 230 feet (70 meters) of the accident. Even if they knew in what direction to run away from the approaching cloud, these victims inevitably would have suffered some serious consequences because of the high concentration of the gas.

The wind and low humidity helped some. Within 5 minutes the vapor cloud had dispersed, and in just 2.5 hours the release measures were normal. But the initial density had been enough to cause the deaths and serious injuries.

 

Why It Happened

According to the accident report of the National Transportation Safety Board (NTSB), the accident was caused by:

  • Excessive vehicle speed
  • Lateral surge of liquid in the partially loaded tank truck
  • Failure of the bridge rail to contain or redirect the vehicle

Lessons Learned

The Hazardous Materials Transportation Act (HMTA) of 1975 had been established to protect the public from the possible risks of transporting hazardous materials, but primarily to stop illegal dumping. The Act’s guidelines for carriers included use of placards, registering with state and federal agencies, maintaining records of pick-ups and deliveries, having a suitable manifest on hand, maintaining rapid response plans, undergoing safety training programs, complying with packaging standards, delivering only at permitted facilities, and reporting and cleaning up any spills or discharges.

Clearly, though, the Act did not adequately address primary contributors to this accident as outlined in a 1979 NTSB investigation report, which concluded that “…DOT-mandated safeguards were not effective in reducing the casualties…” A letter to the Department of Transportation, summarized the Safety Board’s areas of investigation:

Cargo tank safeguards (mandated by DOT) did not prevent abrupt release of the contents of the tank; exposed persons had minimal time to determine their danger and react before becoming engulfed by ammonia

Emergency communications regulations played no role because information was obscured by the sudden cloud around the crash site and because casualties occurred so quickly after the release

Evacuation recommendations in the National Highway Traffic Safety Administration’s Emergency Action Guide for Hazardous Materials, if followed, would have caused some people who survived uninjured to have been lead directly into the oncoming ammonia gas cloud

Routing of hazardous materials contributed to the crash occurring, difficulties experienced by some victims, and to the complexity of rescue problems

To improve survivability during such accidents, the Safety Board’s conclusions and recommendations emphasized the importance of collecting survival action data and incorporating that information into future revisions of the DOT Materials Transportation Bureau’s programs, procedures, and guidelines. Also, sharing the lessons learned with other public and private emergency response safety program managers would benefit their response activities with minimum delay.

Some Final Thoughts

Certainly accidents before and after this one involving releases of ammonia or other hazardous materials have had much more devastating results. But this 1976 ammonia release is memorable for the sheer drama of it, such as the location—so close to residential areas, major office buildings, and a new, upscale retail center. The West Loop had been completed only eight years before (1968). Yet the barriers evidently were not designed or constructed to contain or redirect the vehicle. The Hazardous Materials Transportation Act of 1975 did not address engineering or construction factors such as tanker safety or barriers, and if it had, one year's time is not enough to expect full compliance.

In 1997, US Senator Kay Bailey Hutchison reported that the United States transports more than a billiion tons of hazardous materials by land, sea, and air each year. This was an estimated 500,000 shipments each day. She acknowledged the industry's overall safety record but also said that 8 to 15 deaths per year were attributed to transportation of hazardous materials, and most fatalities involved truck accidents wherein petroleum products ignited. At that time, Sen. Hutchison, representing Texas, chaired the Subcommittee on Surface Transportation and Merchant Marine. Her statement was delivered to a hearing to reauthorize the Hazardous Materials Transportation Act. That did not happen in 1997 but did in 2003.

Nondestructive testing techniques structural defects in tank cars, increasing rollover protection requirements, and increasing crashworthiness of front heads on cargo tanks were on the 1997 hearing agenda for Robert Chipkevich of the NTSB.

Although progress has been made, such accidents still occur. Fatalities, injuries, environmental harm, and lost assets are still too frequent.

References

Image Credits

Also view the YouTube video: Anhydrous Ammonia Accident - Houston, Texas 1976 (3:00 min)
This post is from IRC's October 2009 bulletin. >>Read more
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