Studio Roosegaarde's Urban Sun proposal to use far-UVC lights to make outdoor spaces safer from Covid-19 can only be used at night and people still need to wear masks and observe social distancing, the designer has explained.

The studio stands by the claim stated on its website that Urban Sun "cleans public spaces of coronavirus" but added that it is "not intended to be an ultimate solution".

It admitted that the concept, which proposes using far-UVC radiation to neutralise airborne viruses in outdoor spaces, can only be used after sunset. "We are not claiming that Urban Sun prevents infection for everybody," the studio said.

Following the launch of Urban Sun this week, Dezeen readers posed questions about the concept in our comments section.

"If the light is safe why not sell light bulbs we could use everywhere?" wondered Steve Hassler. "Is this built?" asked SK. "Does it come with a smoke machine?" queried Rd. A representative of Studio Roosegaarde has responded to all the comments.

To find out more, the Dezeen team sent a list of technical questions to the Dutch studio about its outdoor lighting proposal. Here are the studio's responses:

Dezeen: Did the launch in Rotterdam feature a working prototype of Urban Sun?

Studio Roosegaarde: Yes, it did! The far-UVC lamp is visible in the video too.

Dezeen: Is the cone of yellow light shown in the images just regular light? Is this to give a sense of the (invisible) far-UVC light?

Studio Roosegaarde: Yes. The visible ring demarcates the edge of Urban Sun. Far-UVC is invisible to the human eye, so we created a means by which the space could be visualized.

Dezeen: All the images show the product in use at night. Presumably, the yellow light would be invisible during the day. How will people know where the safe area is?

Studio Roosegaarde: This is an excellent question. For now, Urban Sun is focussed on evening gatherings, right after sunset, for this very reason.

Many of the logistical issues surrounding the implementation of Urban Sun are ones that would need to be discussed based on the site specifications. Urban Sun has, and will continue to, evolve as minds from the scientific, design, cultural, and governmental communities are involved in its development.

Dezeen: Most Covid-19 transmission takes place indoors. Transmission outdoors appears to be extremely rare. Why is this product designed for outdoors rather than indoors?

Studio Roosegaarde: Urban Sun is designed to make human gatherings safer. It was situated outside for the initial launch to create a sense of place and a vision of possible other locations. Urban Sun can also be applied indoors, but additional modelling would need to take place.

Even though outdoors is said to be safer (even science does not state this as a fact), currently all large scale events are prohibited. Urban Sun is intended to create an additional layer of safety on top of current government regulations. Any improvement that makes a space safer in these times is a positive one.

Dezeen: Air circulates far more freely outdoors than indoors. The air in the cone beneath Urban Sun would be constantly changing. How can you ever be sure that you can sanitise the air in the cone to any kind of safe degree?

Studio Roosegaarde: It is a "safer degree". We never say "safe degree" since conditions can change. By neutralizing aerosol-borne viruses, there are fewer opportunities for them to spread. Shifting air will result in ongoing sanitizing of viruses within the cone. Each Urban Sun will therefore have a site-specific design, which includes elements such as wind and flow of people. Fine-tuning this design creates an optimum impact.

Dezeen: Has Urban Sun been tested? If so please describe the test and the results.

Studio Roosegaarde:  The efficacy of Urban Sun has been modelled by Meg Science, an independent photobiology lab based in Italy. The measurement and calibration of the actual far-UVC wavelength of Urban Sun was conducted by the Dutch National Metrology Institute VSL in Delft. Urban Sun also meets the International Commission on Non-Ionizing Radiation Protection (ICNIRP) safety guidelines for ultraviolet light.

In an interview on Dutch Public Radio NPO1, renowned independent virus experts Ab Osterhaus, Andreas Voss, and Ivo Lede, who were not involved in Urban Sun, expressed the value and effect of Urban Sun. More exhibitions and their results will be shared in the nearby future.

Dezeen: When Dezeen interviewed Daan in January, he said it takes two minutes for Urban Sun to sanitise a space. Apparently, this refers to a previous iteration of the design. What was the previous spec and why was the spec downgraded?

Studio Roosegaarde: Reduction of the viral load by far-UVC light is not a linear progression. Initial reduction is very high, and then the rate of reduction increases at a decreasing rate as it approaches 100 per cent. The size of Urban Sun, as well as the space within which it is placed dictates, the specifications.

In this case, the project was not downgraded but recharacterized to err on the side of caution as we are aware not only of the importance but also the sensitivity of projects relating to public health.

Original modelling took place in a courtyard in London which was blocked from the wind, increasing the efficacy of Urban Sun. Ultimately, the launch occurred next to a windy river in Rotterdam. Changes reflect the change in location.

Dezeen: If it takes several minutes to neutralise viruses within the air cone, how can it prevent infection? It two people are standing close together, the virus could pass from one to the other within that time before being destroyed.

Studio Roosegaarde: To emphasize again, we are not claiming that Urban Sun prevents infection for everybody. Urban Sun creates a space for safer human gatherings. Government regulations and advisories regarding distancing still need to occur within Urban Sun. Urban Sun reduces risk. It does not eliminate it.

Dezeen: What about blind spots? If someone is standing in the shadow of another person, presumably they won't be protected?

Studio Roosegaarde: Additional modelling will be necessary and blind spots are a possibility. Urban Sun is not intended to create a guaranteed safe space. It is intended to create a safer space. Precautions regarding the coronavirus include masks, distancing, frequent temperature checks and testing.

None of these is a failsafe but combined have the power to reduce risk. Urban Sun is an additional layer of safety, to be added, not to replace.

Dezeen: How high above the ground does Urban Sun have to be? The renderings appear to show it around 10m above ground level.

Studio Roosegaarde: The necessary height of Urban Sun depends on many factors including intended coverage area, wind, capacity, and structures within or surrounding the space. Installations of Urban Sun will need to be evaluated based on these and other factors.

Dezeen: How much will it cost to install each Urban Sun?

Studio Roosegaarde: The cost of Urban Sun will be determined by many factors including location, desired coverage scope, and site-specific requirements. For detailed proposals, we request that interested parties email [email protected].

Dezeen: What area could each Urban Sun keep safe from Covid-19?

Studio Roosegaarde: Again, it "is safer", not "safe". This will depend on the height and size of the Urban Sun and site-specific details. To reiterate, Urban Sun will provide an additional layer of protection, it is not intended to be the sole source of protection, nor should it be applied in contravention of current government regulations.

The Urban Sun in Rotterdam is around 100 metres squared. The larger design is 3,500 metres squared and we are working on much smaller and bigger versions. A toothbrush prevents cavities but does not eliminate them; it is dependent upon the circumstances of your tooth care (products, frequency, etc). The same applies to Urban Sun; it is site-specific. Optimal results depend on situational detail. Modelling for each exhibition will be made public, as per our commitment to transparency.

Dezeen: There seems to be uncertainty as to whether far-UVC is safe for humans. Is this technology approved for this kind of use public by any regulatory authority?

Studio Roosegaarde: The measurement and calibration of the actual far-UVC wavelength of Urban Sun was conducted by the Dutch National Metrology Institute VSL in Delft. Based on that, it can be stated Urban Sun meets the International Commission on Non-Ionizing Radiation Protection (ICNIRP) safety guidelines for ultraviolet light.

Our group of scientists and experts have supported this statement. Urban Sun is supported by the Council of the Public Health & Society Board, the Netherlands’ independent parliamentary advising body.

Urban Sun is based on several peer-reviewed scientific studies by Columbia University and Hiroshima University. Many companies are exploring uses of far-UVC light in treating aerosol-borne viruses. Urban Sun is intended to serve as a call to action to all societal actors, including governments, to explore the potential of this technology and design.

Dezeen: Where does far-UVC sit on the UV spectrum? I haven't been able to find a diagram that explains this clearly. [TBC]

Studio Roosegaarde: Several images are available online if one googles "the spectrum of visible and non-visible light". Far-UVC sits on the far left of the spectrum, visible light in the middle, and infrared all the way to the right.

Dezeen: Your press text says that far-UVC is "new". What do you mean by this? Surely far-UVC has always existed?

Studio Roosegaarde: New as in compared to traditional UV, which has been in use for more than 80 years. The industry, therefore, describes 222nm [the wavelength of far-UVC light] as "new" since the science community got more involved in evaluating its use and efficacy starting in 2018.

Dezeen: Your press text says that far-UVC can "safely clean up to 99.9 per cent of the coronavirus". Are you claiming that Urban Sun can do the same? Does this refer to the entire light cone beneath the source?

Studio Roosegaarde: Urban Sun explores potential. Far-UVC can safely clean up to 99.9 per cent of viruses from the air, including the coronavirus. Given optimal circumstances, Urban Sun may do so. Further research is needed, and Urban Sun should be explored as a potential tool for providing an additional layer of safety.

Dezeen: The intensity of UVC radiation diminishes over distance from the source. What percentage of Covid-19 does Urban Sun neutralise at ground level? How long would it take to achieve this?

Studio Roosegaarde: Urban Sun design is focused on the layer around the human body and face height to have the optimal amount of far-UVC light to reduce viruses. This includes factoring in the diminishing of light through distance.

Dezeen: Do you think the release of this product is helpful to the debate around Covid-19 transmission and to public understanding of this complex topic?

Studio Roosegaarde: We believe that people are suffering greatly as a result of social isolation. Many measures have been introduced to allow for our basic needs to be met; masks must be worn in shared spaces, in the Netherlands all restaurants and bars are closed (except for takeout) and there is a curfew in place from 21:00.

We believe that pursuing an idea that creates an additional layer of safety creates opportunities for safer human gatherings, an emotional and social necessity that was abruptly cut off a year ago. Discussions about the repercussions of this complex topic are ongoing all over the world; how does social isolation impact the old, the young, the poor, the middle class? Urban Sun is an opportunity for us to discuss what possible solutions might look like, now and in the future.

Dezeen: Do you stand by the claim that Urban Sun "cleans public spaces of the coronavirus"?

Studio Roosegaarde: Yes. As we have emphasized, Urban Sun is not intended to be an ultimate solution. It is a starting point, a first step towards a better normal. A car has both brakes and seat belts, why should precautions and measures related to viral transmissions be limited to a single solution?