The ancient city of Pompeii was decimated in 79 CE when the volcano Mount Vesuvius erupted, killing thousands of people in the city and nearby Herculaneum. Now, a new study suggests it took just 17 minutes for the ash and gas released to wipe out the doomed inhabitants.

The estimated 2,000 people who perished were not caught escaping rivers of lava, but died from asphyxiation from the giant cloud of scorching ash and gas ejected by the volcano – the debris from which later covered their bodies, preserving their postures and thus marking the start of the mystery that would long inspire researchers to get to the bottom of what really happened that day.

Herculaneum, at the bottom of the volcano, didn’t stand a chance. However, Pompeii, 10 kilometers (6 miles) away, may have had more survivors if the pyroclastic flow from the mountain had lasted less than 17 minutes, researchers suggest in a new study in Scientific Reports.

The preservation of the victims of Pompeii – plaster casts of the indentations of the bodies left in the ash, rather than the preserved bodies themselves – has offered clues into how they died, and theories have ranged from asphyxiation to being flash-heated to death with no time to suffocate, to being baked alive in a more prolonged and grisly death.

The new study looked at the duration of the pyroclastic flow – the dense cloud of ash, gas, and volcanic glass – that reached the city and how fast the particles traveled. The researchers’ model found that the cloud, which had a temperature of over 100°C (212°F), likely engulfed the city for 10-20 minutes after the eruption.

Previous studies have suggested that the poses of the people caught suspended in action as they went about their lives showed that they didn’t suffocate but instead died from extreme heat, their bodies shocked into instant rigor mortis, with temperatures reaching 300°C (570°F) killing people in a fraction of a second.

The new study points out fragments of clothing survived and were not burned by the pyroclastic density currents (PDC), indicating a temperature below the decomposition point for fabrics such as silk and wool, which are 130°C and 150°C (130°F and 302°F), respectively.

Fine ash from volcanic eruptions can travel a long way, and these particles suspended in the air can be very harmful to human health, even in small amounts. According to the researchers, humans can survive pure hot air at 200-250°C (392-482°F) for 2-5 minutes, but inhalable hot fine ash hugely reduces that time. Exposure time is key, they say, pointing to a previous study that showed people who experienced the Merapi volcano eruptions in Java in 1994 and 2010 survived being exposed for just a few minutes. However, this is the first time the duration of pyroclastic density currents has been quantified using Pompeii as a model.

Their findings suggested the city would have been engulfed in the ash, gas, and volcanic particles for between 10-20 minutes, with the average duration of the flow being 17 minutes – long enough to make breathing the hot ash swept up in the currents lethal. “This duration is quite long when compared to the couple of minutes considered as a survivable time for people engulfed in a PDC, even at low temperature,” the authors write.

Understanding the rate and duration of pyroclastic flows from volcanos could be crucial for planning evacuation methods and policies for inhabitants living in close proximity to volcanoes prone to eruptions.