We have reached a strange point in time when it is rare to go on a trip to the supermarket or walk through the park without seeing people wearing surgical-style face masks.

The official advice about whether you should wear these mask varies from country to country, with the British government, for example, not advising the use of face masks for the general public, but the authorities in certain provinces of China making it compulsory.

The World Health Organization (WHO) recommends that, provided you are healthy, you do not need to wear a mask. People should, however, wear masks if they are caring for someone infected with the new coronavirus, or if they are unwell and coughing or sneezing.

There are a number of different masks available, ranging from basic cloth masks and surgical masks to higher-grade respirator masks.

Different masks are recommended for different risk levels. For instance, doctors seeing patients will only need to wear a surgical face mask for general consultations.

If they are carrying out aerosol-generating procedures, such as intubating (putting in a breathing tube), extubating (taking out a breathing tube), endoscopy (inserting a flexible tube with a light and camera) or bronchoscopy (a tube passed down the throat to view the lungs), then respirator masks are required.

The two main respirator masks used are the FFP3 (filtering face piece) and the N95 (not resistant to oil, filters 95 percent of airborne particles). These types are universal.

A surgical (non-respirator) face mask is loose-fitting and usually blue. It creates a barrier to airborne contaminants, whereas the respirator masks form a seal around the nose and mouth and have an efficient filtration system.

Despite discrepancies between different countries' advice, all seem to agree that the respirator masks should be reserved for medical front-line staff only, and are not necessary for public use.

Which masks should the public use?

Many nations, including China, Hong Kong and Singapore, have taken the advice of WHO and advocated the use of surgical masks by those who have symptoms.

Other countries, such as the United Kingdom and Germany, have said if you have symptoms, you should be self-isolating at home and, therefore, there is little need to wear a mask other than to protect anyone that must enter your house, such as a carer.

Interestingly, while the United States Centers for Disease Control and Prevention (CDC) also does not specifically advocate the use of surgical masks, it does advise the use of "simple cloth face coverings" made from common household materials to slow the spread of the virus and prevent people who may have the virus and do not know it - known as "asymptomatic shedders" - from transmitting it to others.

Germany has followed suit. On April 1, the Robert Koch Institute (RKI), the German federal agency responsible for disease control, updated its advice to include wearing temporary textile masks when entering public places where it is not possible to maintain a safe distance of more than 2 metres (about 6 feet) from others. This includes public transport, grocery stores and workplaces.

This, too, is aimed at protecting the public from asymptomatic shedders. The RKI advises that the cover should be worn tightly over the mouth and nose, must not be touched with the hands by the wearer or anyone else while it is being worn, and must be changed if it gets wet.

There is a significant difference between the loose-fitting, usually blue surgical masks used by healthcare professionals and those that are made at home from household textiles.

Surgical masks require a layer of fabric made from an extremely fine mesh of synthetic polymer fibres for air filtration. Cotton is less dense and will, therefore, not be as efficient in filtering.

Who do masks protect?

There have been no studies or trials testing the efficacy of makeshift cloth masks that the CDC recommends for the general public.

There has, however, been a study of the use of cloth masks compared with surgical masks as "standard practice" for preventing influenza-like illness in healthcare staff.

More Doctor's Notes:

This study found that cloth masks were the least effective but there was no true control in this study as the "standard practice" usually involved wearing a surgical face mask.

A robust study usually requires a control element to compare to, and that control element is often "what would happen if you did nothing" or, in this case, wore no mask at all.

But in this study, the control element was "standard practice" which means wearing a surgical face mask in some of the instances. Therefore, the scientists conducting the study were not able to truly compare the effect of a cloth mask or a surgical mask against not wearing anything.

The RKI states that, by covering the mouth and nose, cloth masks can trap infectious droplets that are expelled when the wearer is speaking, coughing or sneezing and, therefore, theoretically reduce the risk of infecting another person.

It also makes clear, however, that this protective effect has not yet been scientifically proven, though it appears plausible.

Can masks protect the wearer?

The RKI also says there is no evidence for self-protection, which essentially means that wearing a face mask is designed to protect others from you, rather than you from others.

There is, therefore, no significant evidence yet that cloth masks do much to protect wearers from catching coronavirus. Furthermore, the problem with members of the general public buying and using surgical masks instead is that there is then a significant shortage for those who need them the most - healthcare workers on the front line.

In March, WHO highlighted this shortage and called for a 40 percent increase in the production of protective equipment, including face masks.

There is evidence that using medical-grade masks can protect the wearer from catching the virus, in the right healthcare settings, but little to support it in the general public.

The most robust trial was a 2020 systematic review comparing surgical masks with respirator masks and no masks in the transmission of influenza.

This found there was a benefit in wearing a mask over no mask at all, but that there was no statistically significant benefit in wearing the specialist N95 masks over a surgical mask.

Previous laboratory studies had shown that N95 masks do provide greater protection. Therefore, experts suggested, when the specialist masks are taken out of the lab and into the real world, they are less effective - possibly because people are not using them properly as they can be very uncomfortable and painful at times.

The results of other studies have mimicked these findings. A 2010 systematic review of surgical masks and respirator masks in influenza epidemics, again found some benefit to wearing a mask, but only for those with respiratory symptoms. They found no benefit for those without symptoms, suggesting wearing a surgical or respirator mask during illness will protect others, but there is less evidence to support wearing them to prevent infection from others

This, therefore, suggests that masks provide some protection when it comes to transmitting the disease to others, but not so much when it comes to protecting the wearer from catching the virus.

The most recent study, published in April 2020, which is still in pre-print, examined the use of masks with a number of respiratory illnesses, including that of coronaviruses, and found again that there was no strong evidence to support the use of face masks by the general public.

While there may not be any significant evidence supporting the use of face masks by the general public, a lack of evidence is not necessarily evidence itself.

Are there downsides to wearing a mask?

Aside from the risk that the public buying masks will deplete the supply chain for those who really need them on the front line, there are some other negatives associated with the public wearing masks.

There are concerns that you may do more harm than good by wearing a mask, as you may not wear it properly and it may increase the chance of you repeatedly touching your mask and face.

The other concern is that wearing a mask may breed complacency by giving a false sense of security, leading to people disregarding other measures that we do have strong evidence for, like handwashing and social distancing.

While we await further trials to give us more robust evidence about the use of masks, ultimately, if you have symptoms, you should stay at home, self-isolating, and not go out in public potentially spreading it further.

If you are asymptomatic and are concerned you might be spreading it unknowingly, then you are unlikely to be coughing and sneezing, and therefore wearing a mask to protect others may not be necessary, especially if you are adhering to the guidelines to remain more than 2 metres away from others.

Remember that these textile and surgical masks are primarily designed to protect the environment from the wearer, not the other way around.