A mechanical engineer explains why cruise lines are so prone to spreading the coronavirus and why hospital ships are safer

US NavyUS Navy hospital ship USNS Comfort.
  • Outbreaks of COVID-19 on cruise lines like the Diamond Princess raise questions about how hospital ships will be any better at containing the novel coronavirus.
  • Two hospital ships are set to dock in New York and Los Angeles as US caseloads surge.
  • Mechanical engineer, Qinyang Chen, said that the ventilation systems onboard hospital ships make them equipped to contain the virus better than commercial cruise liners.
  • President Trump suggested that decommissioned cruise lines could be an additional asset if hospital ships aren’t enough, which may be a viable option if ventilation issues are addressed, said Chen.
  • Visit Business Insider’s homepage for more stories.

Luxury cruise ships like Carnival Corporation’s Diamond Princess have proven fertile ground for the spread of COVID-19. In contrast, two naval hospital ships -if operated with certain precautions – should safely halt transmission, says Qinyang Chen, a professor of mechanical engineering at Purdue University.

The 1,000-bed floating hospitals are set to dock in New York and Los Angeles in the coming weeks. Chen told Business Insider that the two ships’ air circulation systems are equipped to trap and eliminate virus particles, which didn’t happen on the Diamond Princess.

What went wrong on the

Diamond Princess

Chen says researchers can’t prove how more than 700 passengers aboard the Diamond Princess came down with COVID-19. The virus could have spread through handshakes or coughs – even through tiny drops of toilet water spewed into the air when someone flushes. But after a quarantine confined most passengers to their rooms, how did the virus keep finding new victims? Chen suspects the air conditioning system.

Cruise ship cabins are vented with a mix of safe outdoor air and questionable indoor air. “You do get the fresh air from the sea,” says Chen. That ocean breeze should be free of the coronavirus. But “part of the air supplied to each room is returned back to the air conditioning system.” According to Chen, the danger lies in recirculation.

Each time the air circulates, it flies through a coarse filter that snags particles larger that 5,000 nanometres – that’s about the size of your red blood cells. But Chen says the coronavirus is likely dozens of times smaller, rendering the filter an open door rather than a blockade. “Those small particles that might contain the coronavirus are returned to the cabins,” says Chen. So the coronavirus could have spread between rooms, even if everyone followed the quarantine guidelines.

In the end, one in five (19.2 per cent)Diamond Princess passengers tested positive for COVID-19. The high infection rate raises questions about the safety of using ships to expand hospital capacity as the US caseload surges.

Navy hospital ships to the rescue?

The US Navy is readying its two hospital ships for civilian service in April. At the request of local officials, the USNSComfort will set sail for New York City, while the USNSMercy will chart a course for Los Angeles.

The floating hospitals won’t admit COVID-19 patients – instead they will serve as intensive care units to treat traumatic injuries or heart attacks. That way, brick-and-mortar hospitals can focus on patients with the coronavirus.

But this use of hospital ships is controversial. A spike in COVID-19 cases – especially in New York City – means the infection could easily find its way onto the hospital ships.

If that happens, Chen is confident that the naval vessels can contain the disease far more effectively than luxury cruise liners. “Navy ships should be able to sustain biological or nuclear war,” says Chen, adding that the ships were likely outfitted with HEPA filters that catch tiny virus particles. (The US Navy did not reply to Business Insider’s request to confirm this.)

HEPA filters are also used on aeroplanes, where Chen says you are unlikely to catch a virus from recirculated air – though contaminated tray tables or a sneezing neighbour could pose a risk.

In addition to HEPA filters, the floating hospitals’ ventilation systems should avoid mixing outdoor and indoor air. “I recommend that those ships use 100% outdoor air,” says Chen.

With those measures in place, he thinks the likelihood of COVID-19 spreading on the hospital ships is much lower than on cruise liners. Still, he cautions that proper operation of ventilation systems is key, along with screening everyone who boards the ship for signs of infection.

What if two hospital ships aren’t enough?

Last week President Trump floated the idea of converting luxury cruise ships – which are out of commission right now – into floating hospitals if the Navy vessels prove insufficient. The proposition may seem ill-advised given cruise ships’ poor track record on virus containment.

But Chen says the ships could limit spreading by using only outdoor air for the ventilation system. At present, cruise ships mix outdoor and recirculated air to reduce energy costs (the recirculated air doesn’t typically need heating or cooling).

For now, Trump says there are no specific plans to deploy the cruise ships.

Chen believes the COVID-19 crisis could permanently change how luxury cruise liners operate. Beyond shifting to outdoor air for ventilation, Chen says “in the long run they may want to shift to HEPA filters.” The high-efficiency filters are more expensive to operate, but given the precipitous drop in Carnival Corporation’s share value, the investment seems worth the cost.

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