Understanding mould. Causes, problems and solutions

Mould is the historical enemy of our houses. We think we have eliminated it, but here it systematically reappears! If you too have had to deal with this organism at least once, you will find answers to your doubts and some tips on how to get rid of it in this article.

The term 'mould' is commonly used to identify various organisms belonging to the kingdom of Fungi (or Mycota). They generally consist of thin branched filaments called hyphae, which are derived from the germination of fungal spores. The hyphae together form the so-called mycelium, which allows us to detect even with the naked eye the presence of mould on a surface.

The Kingdom of Fungi is subdivided into four Divisions (or Phyla): Zygomycota, Ascomycota, Basidiomycota and Chytridiomycota to which is then added the group of Deuteromycota, also called "imperfect fungi" because, unlike the others, they reproduce asexually. Moulds do not belong to a precise division as there are various species and depending on their morphological and reproductive characteristics they can belong to one of the Phyla mentioned above.

Moulds can be recognised by their characteristic odour and 'spongy' appearance, which can be of various colours: green, yellow, orange, brown, white, grey or black.

As we have already mentioned, mould reproduces by means of spores that are produced by specialised fungal structures. The life cycle of a spore consists of several phases: after formation and maturation, most spores enter a period of dormancy followed by germination. This consists of the rehydration of the cell, which resumes its metabolic activities, subsequently giving rise to the hypha.

The spores are very resistant to both low and high temperatures but only germinate under suitable environmental conditions - different from species to species - and in the presence of a substrate capable of supporting their growth. The optimum temperature is between 20 and 30 °C, but some moulds can also live at higher or lower temperatures. For each fungal species there is a minimum, an optimum and a maximum temperature for development. Outside this range the hyphae do not grow but retain viability and may resume development if conditions become favourable again.
The optimum pH is acidic, between 4 and 6, but there are species that can also grow in a wider range between 2 and 9. For all moulds, the availability of water is crucial, which is why their development is determined by the relative humidity in the environment.

Spores are mainly dispersed in the air in summer and autumn and can grow both inside and outside homes, especially where excessive moisture is present. Poor ventilation is one of the factors that allow them to grow indoors and they can also proliferate in air conditioning systems that are not cleaned regularly.

As we have said, mould can generally form in buildings due to excessive ambient humidity, which may be due to poor ventilation, insufficient thermal insulation or water infiltration. In the first two cases we speak of condensation dampness, which forms most frequently on perimeter walls and at thermal bridges, especially in areas such as the kitchen and bathroom. Mould can also form behind furniture, in showers and bathtubs, around windows and in tile joints, even colonising mattresses, upholstery, clothing, books and food.
The formation of mould can also be favoured by incorrect insulation of the building foundations, which generates so-called rising damp, and by water infiltration due, for example, to broken pipes or poor roof insulation.

It is therefore important to design a building correctly and apply the most suitable construction techniques to avoid the formation of fungal colonies. It is equally important to carry out proper cleaning and a daily air exchange of rooms, together with regular maintenance of air conditioning and ventilation systems.

Moulds can cause not only an unpleasant smell in rooms, but also aesthetic damage to surfaces: they are part of the so-called 'chromogenic fungi', as they do not cause serious damage to structures, but only superficial colour changes.  In the long term, however, they can cause pulverisation and flaking off of plasters. In addition, the airborne spread of fungal spores can cause allergies or asthma in humans.

The types of micro-organisms present in the outdoor air are numerous but are usually in low concentrations, as they are subjected to self-purification mechanisms such as UV rays, oxygen or rainwater. In indoor environments, on the other hand, the concentration is much higher and the proliferation of germs can be favoured by ideal microclimatic situations (humidity and temperature). It should be borne in mind that the growth of microorganisms can also be caused by air conditioning systems or humidifiers that are not properly maintained.

When assessing biological pollution of indoor air, it is important to evaluate the relationship between the indoor and outdoor atmospheric environment regarding the concentration of biocontaminants.
 Fungal spores usually originate from the outdoor environment, but can find ideal conditions for taking root and developing in indoor environments with damp walls or water stagnation: in these cases the concentration of spores can increase considerably.

Pathogenic germs in the air reach humans primarily by inhalation and secondarily by contact and ingestion. The resulting health problems can manifest themselves with gradual severity, ranging from a simple inflammation of the eyes and the first airways (rhinitis, sinusitis) to more serious syndromes such as allergic alveolitis or bronchial asthma.

The fungi most responsible for allergic manifestations belong to the genera Alternaria, Cladosporium, Aspergillus, Candida, Penicillium, Mucor, Fusarium and Rhizopus. Some types of fungi (Fusarium, Trichocene, Trichoderma) are capable of producing mycotoxins that can cause asthenia, nausea and gastrointestinal disorders.

The Italian Ministry of Health has published outlining measures to reduce exposure to mould:

• ensure that external walls, foundations, attics and the attic are insulated and well ventilated;
• maintain a humidity inside the home of less than 50%;
• do not leave clothes hanging out to dry for a long time in closed, poorly ventilated rooms;
• Try to eliminate condensation and, if condensation is present, ventilate the room frequently;
• eliminate mould stains with specific products;
• properly maintain humidifiers, air conditioners and mechanical ventilation systems;
• as far as possible, keep furniture away from the wall;
• regularly wash shower curtains, sanitary ware and bathroom and kitchen walls.

Moulds often appear even if steps are taken to avoid their formation and should be eliminated as soon as one becomes aware of their presence.

There are curative methods, which put an end to the infestation, and preventive methods, which aim to prevent the spores from developing and colonising the substrate.

Various methods and remedies can be found on the web, but not all of them are really effective. Certainly the ones that are most effective against mould are products based on sodium hypochlorite. Sodium hypochlorite is in fact a biocide active ingredient in its own right, as can also be read on the Echa (European Chemicals Agency) website.

If the attack is mild and not too extensive, the use of classic laundry bleach may suffice. You can dab the surface with a cotton ball or sponge soaked in the product and after a few minutes the mould stains will disappear. It is better not to rub the surface to avoid spreading any spores that may still be alive. In the case of more extensive and deeper attacks, it is best to use specific products designed for the various materials.
Lignum Biosel, for example, is an excellent remover for mould on wood. It is generally not advisable to remove mould dry, using brushes or other tools, as this encourages the dispersion of spores and the proliferation of germs.

Other substances talked about on the web:

• hydrogen peroxide has bleaching properties and is considered a biocidal active ingredient as it is used as a disinfectant. It can be used as an alternative to sodium hypochlorite, especially at high concentrations, but must be handled very carefully;
• ammonia is used in detergents for its degreasing power, but has no biocidal action;
• acetic acid (contained in vinegar) is known for antibacterial properties, but is not a biocidal active ingredient;
• sodium bicarbonate has cleaning properties, but is not a biocidal active ingredient;
• borax (sodium tetraborate) is an approved biocidal active ingredient in PT8 (wood preservatives) with a preventive and curative effect on fungi and insects. For more on boron salts you can read our article.

All the substances listed - with the exception of sodium bicarbonate - have specific hazards, which may vary depending on the formulation of the final product.

It is good to remember that the simple removal of mould does not solve the problem! Once the surfaces have been cleaned, it is possible to apply specific anti-mould products that have a preventive action. It is also good to put into practice all the suggestions mentioned so far.

We hope with this article we have clarified your ideas on moulds. If you have any other questions and curiosities, please do not hesitate to write to us!

Bibliography

• Funghi e insetti nel legno, A. Gambetta, Nardini Editore
• Degrado biotico del legno, M. Dalprà, Daniela Piazza Editore
• La biologia vegetale per i beni culturali - Biodeterioramento e conservazione, G. Caneva, M.P. Nugari, O. Salvadori, Nardini Editore