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

• Ammonia. Wikipedia. Online at http://en.wikipedia.org/wiki/Anhydrous_ammonia Accessed 26 October 2009.
• Bridgen K, Stringer (December 2000) Ammonia and urea production: Incidents of ammonia release from the Profertil urea and ammonia facility, Bahia Blanca, Argentina 2000. Exeter, UK: Greenpeace Research Laboratories, Department of Biologica. Sciences, University of Exeter. Online at http://www.greenpeace.to/publications/profertil report.pdf accessed 15 October 2009.
• Committee on Acute Exposure Guideline Levels, committee on Toxicology, National Research Council. Acute Exposure Guideline Levels for selected Airborne chemicals. Vol 6. Washington, DC: The National Academies Press, 2008. Online at http://www.nap.edu/openbook.php?record_id=10218&page=65 Accessed 20 October 2009.
• Devine J. (23 June 2008) 1976 Ammonia spill of Houston. Ezine articles. Online at http://ezinearticles.com/?1976-Ammonia-Spill-of-Houston&id=1270104 Accessed 15 October 2009.
• Highway Accident Report: Transport Company of Texas Tractor-semitrailer (Tank) Collision with Bridge column and Sudden Dispersal of Anhydrous Ammonia Cargo I-610 at Southwest Freeway, Houston, Texas May 11, 1976. NTSB Number HAR-77-01, NTIS Number PB-268251. Online at http://www.ntsb.gov/publictn/1977/HAR7701.htm Accessed 15 October 2009.
• HMTA Reauthorization Proposed. (Hazardous Materials Transportation Act)(Brief Article)(Statistical Data Included). Chemical Market Reporter. October 29, 2001.
• King JB. National Transportation Safety Board. Letter dated December 11, 1979, to Howard J. Dugoff, Administrator, Research and special Programs Administration [U.S. Department of Transportation]. Online at http://www.ntsb.gov/Recs/letters/1979/I79_14_16.pdf Accessed 15 October 2009.
• National Propane Gas Association (n.d.). Hazardous Materials Transportation Act – Reauthorization (H.R.3). Online at http://www.npga.org/i4a/pages/index.cfm?pageid=783 Accessed 21 October 2009.
• NIOSH Pocket Guide to Chemical Hazards (Ammonia) (September 2005). National Institute for Occupational Safety and Health. Online at http://www.cdc.gov/niosh/npg/npgd0028.html Accessed 28 October 2009.
• S.2936 A bill to amend the Hazardous Materials Transportation Act to authorize appropriations for fiscal years 1990, 1991, and 1992, and for other purposes. Summary as of 8/30/1990–Reported to Senate amended. the Library of Congress. Online at http://thomas.loc.gov/cgi-bin/bdquery/z?d101:SN02936:@@@D&summ2=1& Accessed 21 October 2009.
• Safety and Health Topics: Ammonia (7 august 2003). US Department of Labor, Occupational Safety & Health Administration. Online at http://www.osha.gov/dts/chemicalsampling/data/CH_218300.html Accessed 21 October 2009.
• Section 6 – VI. Health Effects Discussion and Determination of Final PEL. US Department of Labor, Occupational Safety & Health Administration. Online at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=PREAMBLES&p_id=770 Accessed 28 October 2009.
• Transporting Hazardous Materials. (22 October 2007) United States Department of Labor, Occupational Safety & Health Administration. Online at http://www.osha.gov/SLTC/trucking_industry/transportinghazardousmaterials.html Accessed 21 October 2009.
• United States Congress, Senate, Committee on Commerce, Science, and Transportation. Subcommittee on Surface Transportation and Merchant Marine. Hazardous Materials Transportation Act Reauthorization: hearing before the Subcommittee on Surface Transportation and Merchant Marine of the Committee on Commerce, Science, and Transportation, United States Senate, One Hundred Fifth Congress, first session, May 8, 1997. Washington: U.S. GPO. Online at http://www.archive.org/stream/hazardousmateri00marigoog Accessed 21 October 2009.

Image Credits
Grevemberg, Carroll (photo) in Slotboom, Eric (2003) Houston Freeways. A Historic and Visual Journey. Online at http://www.oscarmail.net/houstonfreeways/ebook/Loop61-_72ppi.pdf Accessed 28 October 2009.

Isopleth (Dispersion Map) 28 October 2009. Houston, TX: IRC Risk and Safety, LLC.