Killing mould with light

By - , Build 159

A proven technology using UV light to clean air of contaminants is being used in a BRANZ project looking to harness it to remove mould from New Zealand homes.

Figure 2: Mould on a window drainage channel (left). The channel on the right is coated in titanium dioxide that combines with sunlight to activate the PCO process and stops mould growth.
Figure 1: Mould spores after a few days’ growth. Only plate 1 in the top left corner had been exposed to PCO. Plate 5 was a control.

THE TYPICAL adult spends over 80% of their time indoors so the quality of air inside homes and buildings can have a huge impact on our overall health.

Need to remove indoor air contaminants

Harmful biocontaminants such as allergens, fungi, moulds, bacteria and viruses are easily introduced to indoor air on clothing, skin, food and new furnishings. Even some building materials and furniture can release potentially harmful gases such as formaldehyde into the indoor air.

If not removed from the indoor air, these contaminants can increase the risk of respiratory illness and lead to mould growth in high-humidity environments where fungal spores are present.

Ventilation and filtration insufficient

Ventilation has traditionally been used to dilute the concentration of indoor contaminants with relatively clean outside air. However, it does not necessarily eliminate these contaminants from the indoor air and, in many cases, introduces more irritants such as fungal spores present in the outdoor air.

Filtration systems can filter most of the contaminants before they enter living spaces. Filters generally just remove contaminants from the air and transfer them onto the filter without killing them. If not cleaned or replaced regularly, air filters can become breeding grounds for fungal and bacterial spores.

Light to the rescue

The article Clearing indoor air with light in Build 121, pages 44–45, described an emerging technology. Requiring UV light and a semiconductor such as the white pigment commonly used in paint, food, cosmetics and sunblock, it has been proven to kill a range of harmful biocontaminants on contact.

It works passively, requiring only UV light to stimulate it into action, meaning it can be solar driven with no electrical requirements. Importantly, while it can eliminate harmful contaminants, it remains non-toxic and safe for humans to touch.

Photocatalytic oxidation use growing

This relatively new technology, known as photocatalytic oxidation (PCO), has been appearing in an increasing number of commercial products from heat pumps and ventilation systems to stand-alone air purifiers.

Some manufacturers are even experimenting with including PCO in products such as cement, timber, paint systems and roofing products to passively clean outside air. This is particularly pertinent in cities such as Beijing where outside pollution levels can be dangerously high.

PCO has proven to be effective at removing a wide range of contaminants, not just fungal mould spores. Bacteria, viruses and a growing number of harmful gases such as formaldehyde, toluene and nitrogen oxides, a common component of car exhausts, have also shown potential to be eliminated using PCO.

In several applications, the PCO process can be enhanced by the inclusion of certain elements such as silver or copper particles. Scientists are also investigating solutions to allow PCO to work under room light without the need for UV light.

Because it works passively, there is the possibility of incorporating PCO in common building materials such as interior linings and paint systems that are in contact with the indoor air.

BRANZ found PCO reduced mould growth

A BRANZ research project has begun investigating the potential for PCO to be used in New Zealand and its effectiveness at eliminating mould spores commonly found in our homes.

Current BRANZ research has demonstrated that PCO is effective against the commonly found fungal spore Penicillium chrysogenum under certain conditions. These spores are often found in New Zealand houses growing as mould, particularly when the indoor environment is humid or damp.

Spores were exposed to PCO for various lengths of time and under two different conditions – mixed with dry, sterile dust and having been thoroughly wetted with distilled water. Spores were then transferred onto Petri dishes and left to grow for 7 days (see Figure 1).

Figure 1: Mould spores after a few days’ growth. Only plate 1 in the top left corner had been exposed to PCO. Plate 5 was a control.

Requires higher relative humidity to work

A reduction in mould growth was observed when a PCO process was used compared to simply exposing spores to UV light or keeping them in the dark. This result was easily seen using wet spores but less so using dry spores.

We believe that the dry spores require a higher level of relative humidity in the ambient air than we could achieve in our experiments for the PCO process to have a more noticeable effect.

In a realistic situation, such as a kitchen or bedroom wall, mould is extremely unlikely to grow if the relative humidity remains low. Therefore, an inability to kill dry mould spores using PCO at low relative humidity does not have an impact on the overall potential for this technology to improve the indoor air quality.

We will test PCO effectiveness against mould spores at higher relative humidities in an upcoming round of experiments.

Back to top

Worth exploring further

This simple laboratory experiment demonstrates the potential for PCO to improve indoor air quality in New Zealand and justifies future research.

The next step is to examine the more realistic case of airborne spores within a living space and include some common allergens and known asthma-triggering contaminants. Very little research has been done here or internationally using dry, airborne contaminants due largely to the technical challenges associated with such an experiment.

Figure 2: Mould on a window drainage channel (left). The channel on the right is coated in titanium dioxide that combines with sunlight to activate the PCO process and stops mould growth.

Possible inclusion in building materials

Eliminating mould and other biocontaminants from the indoor living space represents an exciting new direction for BRANZ research. It opens possibilities for collaborating with an array of industry partners as we test the viability of including PCO and other technologies in construction materials where mould tends to form.

Back to top

Download the PDF

Articles are correct at the time of publication but may have since become outdated.

Figure 2: Mould on a window drainage channel (left). The channel on the right is coated in titanium dioxide that combines with sunlight to activate the PCO process and stops mould growth.
Figure 1: Mould spores after a few days’ growth. Only plate 1 in the top left corner had been exposed to PCO. Plate 5 was a control.

Advertisement

Advertisement