Our friend and fellow disaster researcher was recently asked to self-quarantine for two weeks after returning from south-east Asia. Although he hadn’t travelled to China, he had taken a plane home with others who were returning from Beijing. A few days before his voluntary quarantine was set to expire, he visited his office to fetch a library book and ran into a colleague. Why did he take the risk? And was his colleague, who knew about the situation, concerned about contagion?

People are often optimistic about risks. In the field of risk perception, we call this an “optimism bias”; people may think they can control their own exposure to diseases, that they don’t need a vaccine because they aren’t susceptible to flu, or that they won’t transmit their cold to others. Our friend was confident he couldn’t be a vector for coronavirus, or Covid-19; likewise, one of our family members recently flew across the country with a severe respiratory ailment, insisting that her mask and careful handwashing would protect those around her on the plane.

Surgical face masks offer the sense, however illusory, of protection

It’s impossible to be completely certain about the risks of Covid-19. We don’t yet know enough about the length of its incubation period, how the disease is transmitted or what makes some people more susceptible than others to it. And there’s even greater uncertainty about what policy responses are most appropriate – did China’s decision to quarantine entire cities help prevent the spread, or exacerbate it? Is Italy’s decision to fine people caught entering or leaving outbreak areas “draconian”, or necessary?

Uncertainty is the root of many anxieties about the virus. Current estimates suggest that seasonal influenza is a greater public health threat than Covid-19, but our uncertainty about coronavirus – seemingly new, mysteriously contagious and quickly spreading across Europe – feeds into how we perceive risk. If people are overconfident in the protective benefits of any one particular action, they may end up doing things that are risky, or even counterproductive.

The Ebola epidemic in 2014 showed how peoples’ social biases can influence their ability to stay healthy. Some doctors, who wanted to help people as quickly as possible, rushed to aid patients without wearing protective clothing, and subsequently contracted the disease.

Many people in Europe and the US have begun wearing surgical masks to protect against coronavirus. According to experts, the value of surgical masks depends entirely on the context in which they’re used: a surgical mask won’t work unless it’s worn consistently and properly. If worn incorrectly, their utility quickly plummets. We touch our faces, noses and eyes many times a day, making it difficult to completely avoid contact with infection. Though face masks may provide the feeling of security, masks are most valuable in situations where they are necessary – such as among frontline health workers.

One woman we observed recently on a flight was continuously adjusting her face mask, pulling it aside to talk to her husband (who wasn’t wearing a mask), and touching several communal surfaces after rubbing her eyes. If her objective was to immunise herself from Covid-19, she could have directly exposed herself. If she was attempting to protect others from her own respiratory illness, she did the opposite.

And by depleting the global stock of face masks, those in the west – who are currently at low risk of contracting coronavirus – may be using up resources that could be needed elsewhere. The World Health Organization recently warned that stockpiling face masks could lead to a global shortage – hampering the fight against the spread of the virus.

When we’re uncertain about something, we often rely upon our feelings and prior experiences in place of information. Surgical face masks offer the sense, however illusory, of protection. Moreover, a climate of uncertainty can cause misinformation to flourish. This is why clear messaging from trusted sources, and guidance on what to do and how to do it, is essential during a pandemic.

Our perceptions of risk can lead us to view severe policies, such as restrictions on travel, as an appropriate precaution. Yet drastic responses have knock-on effects: China’s economy has taken a huge hit since coronavirus, affecting global markets. People of Asian descent across the world are experiencing unfounded stigma. And austere measures, such as travel bans and quarantines, may impede access to healthcare supplies and resources at the time when they’re needed the most. Rather than prevent coronavirus, our perception of risk may help it spread.

• Ann Bostrom is a professor in environmental policy and Nicole Errett is a lecturer in the department of environmental and occupational health at the University of Washington