ON THIS DAY: May 10, 1924, Pittsburgh’s Liberty Tunnels fill with deadly fumes, 33 people overcome

PITTSBURGH, Pa. — When the Liberty Tunnels opened on March 23, 1924, they marked the beginning of a transformation that would define the South Hills. Heralded as an engineering marvel, they had a potentially fatal flaw that would be revealed just two months later.

The Liberty Tunnel was the first tunnel specifically designed for automobile traffic and remained the longest in the world until the (as yet unnamed) Holland Tunnel opened in New York City shortly afterward.

At the time of its construction, engineers didn’t have a lot of experience in determining how much pollution cars emitted or how much fresh air would need to be pumped in to offset it. The closest estimations for ventilation were based on train tunnels, which didn’t account for stoppages within them, and on research conducted by the Bureau of Mines. Engineers knew they needed ventilation in the tunnels (and the Liberty Tunnels would become the first artificially ventilated tunnels), but the original system was undersized and not operational when the tunnels were completed. There also simply wasn’t a lot of research available to determine how much carbon monoxide a person could breathe before it became dangerous.

After much public debate about the adequacy of the ventilation system’s design, the tunnel opened to traffic soon after the chief engineer of New York’s new tunnel, Clifford Holland, said it was sufficient. Calculations at the time relied on a steady traffic flow that created a natural draft of air in addition to the incomplete and nonfunctional ventilation system. The mistake was compounded by the Pittsburgh Department of Public Works, which had based the Liberty Tunnel’s flow rate on traffic measured on Bigelow Boulevard.

When 3,200 trolley motormen and conductors for the Pittsburgh Street Railway Company went on strike at midnight on May 10, 1924, downtown commuters had no other option than to flood the roadways with cars. At that time, the Liberty Bridge had not been built yet, so traffic exiting the tunnels had to snake down to the Smithfield Bridge to cross the river into downtown, which caused massive delays, even for a Saturday.

It didn’t take long for the congestion to back up into the tunnel itself and, not long after that, the fumes from all the stopped cars began to gather in the stagnant air. The Pittsburgh Press reported that 649 cars jammed into the tunnel.

Hearing horns honking from panicked drivers, police officers scrambled into the tunnels, telling drivers to shut off their engines, but it was too late and drivers began slumping over their steering wheels of carbon monoxide poisoning. Other gasping drivers fled their vehicles, which left them idling and spewing out more exhaust into the growing fog.

Miraculously, no one died in the thick air. First responders fought for more than an hour to evacuate the tunnel and police officers using wet handkerchiefs as gas masks had to be rescued by their comrades when they too collapsed.

A total of 33 people had to be hospitalized for carbon monoxide inhalation.

The tunnel was cleared of vehicles and reopened later that afternoon but with new restrictions. Until the ventilation system was fully operational, police would only allow six cars per minute to enter the tunnel. It took until 1928 for engineers to get the ventilation system capable of keeping the tunnel air clear even when traffic was packed inside.

Today, eight 12-foot-diameter fans are connected to a large brick utility building on Secane Avenue on Mount Washington by 200-foot high shafts. The shafts exit the building into four towering 110-foot-tall square stacks and the system can maintain a 15 mph airflow through each of the tubes.