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As the Covid-19 pandemic has grown, you’ve likely seen photos and videos of workers in protective gear using high-pressure sprays to disinfect city streets. Spain has even taken the radical step of spraying lye on the beaches. You may have wondered if this really makes much difference in transmission risk. If not, why would governments spend time, energy, and dollars to do it?
Based on our knowledge of the conditions required for disinfectants to work, we suspect that these activities are as much about authorities doing something as they are about stopping the spread of Covid-19.
The likely effectiveness of spraying streets and other public places depends on how the virus spreads, how disinfectants work, and under what conditions they are used.
AP
A young girl is sprayed with disinfectants when she arrives to receive free food distributed by social workers in Kathmandu, Nepal.
How does the virus spread?
We now know that the virus spreads mainly in two ways.
The first is through airborne droplets and aerosols that originate from infected individuals. The drops are expelled into the air through a cough or sneeze and can infect another person who finds them at close range. The droplets are larger and do not stay in the air for long, and quickly settle on the ground or other surface.
Aerosols are smaller and stay suspended for longer, up to three hours. Aerosols will dry quickly and disperse over time. This makes it less likely that a person will be exposed to enough viral particles, known as the infectious dose, to become infected.
Read more: coronavirus moves through the air in microscopic droplets: here is the science of infectious aerosols
The second way of spreading the disease is through surface contamination. When the droplets settle, the virus can persist for variable periods, depending on the nature of the surface. For example, one study found that the virus survives up to 72 hours on plastic and stainless steel, eight hours on copper, and four hours on porous surfaces like cardboard.
This experiment, however, was performed under indoor laboratory conditions. So far, there is no information available on how long the virus can survive outdoors. The likelihood that you will become infected when walking on city streets is also unknown.
Read more: We know how long the coronavirus survives on surfaces. This is what it means to manage money, food and more.
How could the disinfectant work?
We must also consider the disinfection process. According to press reports, most authorities are using a dilute bleach solution to disinfect areas of the city. Research suggests that the Covid-19 virus is susceptible to bleach, but requires a contact time of about one minute to be effective.
Even if the disinfectant reaches all exterior surfaces that can be touched by people, including dew-protected areas, there is still a problem with using chlorine in typical outdoor conditions. Sunlight and organic matter buildup on surfaces will quickly deactivate chlorine, the active ingredient in bleach. This means that the disinfectant will likely become ineffective before the virus is removed.
For the virus to infect a person, it needs to enter the body. This can happen when your hands are contaminated by touching a surface and you bring your hands to your face, close to your nose or mouth. But when was the last time you touched the ground and then touched your face without washing your hands?
The average person will seldom come into direct contact with the streets and trails of the city with their hands. That’s another reason that spraying these surfaces with disinfectant is unlikely to be an effective control measure.
Commonly touched surfaces, such as railings and road junction buttons, are more likely sources of infection, but would need to be cleaned before disinfecting with chlorine. This is because organic matter accumulates on frequently touched surfaces, including natural oils on human skin. Even if cleaning is done before disinfection, this process should be continuous, as the next time an infected person touches the surface, it can become contaminated again.
Spraying disinfectant in the air will have the effect of reducing the amount of virus that is suspended as aerosols. However, this will have a very limited effect as the disinfectant will quickly disperse. Aerosols will be reintroduced the next time an infected person travels through the area.
Another consideration is that the bleach drops in the spray can be corrosive and cause harmful respiratory effects when inhaled. Spraying should only be done when there are no people around.
A much more effective regimen is to recommend strict personal hygiene. This includes regular hand washing with soap and water and the use of an alcohol-based disinfectant when hand washing is not possible.
Why, then, are countries spraying streets?
So if spraying disinfectant in urban areas is unlikely to be effective, why are we seeing that some countries are doing this?
Without being aware of the decision-making process, it is difficult to say. However, there are a couple of possibilities. One is that authorities want to create an environment that is free of Covid-19 but is not following science. A more likely reason is to help people feel safe because they see the authorities taking action.
In a crisis, people are less likely to incorporate information that challenges their current beliefs. Although science indicates that urban disinfection is likely to be ineffective, the general public is likely to believe otherwise. As a result, spraying city streets could have the effect of allaying fears and building trust in the government and the messages it distributes.
However, a possible downside to this is that people who feel that their environment is safe may be less strict about personal hygiene and physical distance. These precautions are vital to prevent the virus from spreading throughout the community; If people stop observing these behaviors, the virus is likely to spread much more quickly.
The bottom line of this is that while urban disinfection can increase public confidence, it is likely to be ineffective in protecting the public from infection.
Lisa Bricknell is a Senior Lecturer in Environmental Health at CQUniversity Australia. Dale Trott is a conference on environmental health at CQUniversity Australia.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
