How to Kill Mold

Author: John Fay   Date Posted:6 November 2013 

How to Kill MoldMold is often difficult to remove and is a common problem in the bathroom and other places where there is water or moisture. It is particularly difficult to remove when it’s stuck between tiles. Whether it is in a shower recess, basin, bathtub, or any other surface in the bathroom, you can use bleach, sodium hypochlorite, or a high concentration ammonium compound to kill mold.

Before any method to remove mold using these three options, you must first establish that it is really mold. The first thing you should know is that mold usually grows in damp, unrestricted areas where it can grow undisturbed. This means that it usually grows in hard to reach areas which are almost untouched when one uses the bathroom. This includes the grout, the corners of showers, and in hard-to-reach places. A good way to avoid mold growth in the first place is to keep your bathroom dry when it is not being used.

If it is observed elsewhere, it is probably just dirt and grime which can easily be removed by a degreaser or a strong, generic bathroom cleaner. However, extensive mold growth may allow the spread of mold in those areas as well.

Once you have established that it is indeed mold, you may consider one of the mold killing options mentioned above.

First is using bleach which is the easiest, fastest, and least expensive way to kill mold. One should use bleach with caution because, aside from bleaching the area in which mold has grown, it is not very user friendly as contact on the skin may have adverse effects and the odor may irritate those with respiratory problems. Aside from that, it is the most practical option. If it does not work then it is, again, most likely not mold at all.

To use bleach in killing mold, strong bleach in particular, you should first make sure that the area is well ventilated. Working with open doors or windows, or turning on the exhaust fan is advised as the fumes emitted by bleach are harsh. To avoid contact on the skin, wear gloves.

Mix a solution that is a ratio of one cup of bleach per gallon of water, which translates to 1 part bleach to 10 parts of water. Apply the solution to non-porous surfaces. You may put it in a spray bottle for easy application, or you could opt for putting the solution in a bucket and apply it on the surface with a sponge or a cloth. Rinsing is not necessary as the bleach will avoid further growth of mold. However, if small children or pets are to be exposed to the surface it may be advised to rinse the area to avoid accidents.

There are other criticisms in using bleach, including the fact that it is harsh and corrosive and may damage the surface to be used on. In some cases, the roots of the mold are not actually killed, allowing regrowth. Safer alternatives include borax or vinegar. Remember to use only either bleach or ammonia for killing mold. Mixing the two will emit dangerous and deadly fumes similar to weapons used in World War II.

The active ingredient in bleach that kills mold is sodium hypochlorite, which becomes hypochlorous acid when added to water. The pure form is a clear, slightly yellowish solution. Most domestic bleaches are only 5% sodium hypochlorite. It is also found in many disinfectants and household cleaners, and is largely used industrially for bleaching, disinfecting, and purification purposes.

Most cleaning products, when not containing sodium hypochlorite, contain quaternary ammonium compounds in various concentrations. Quaternary ammonium compounds or “quats” are effective in killing bacteria and some viruses. In this case, those with higher concentrations are used in killing mold. However, they are inactivated when coming into contact with soap or its residues.

Before killing mold using high concentration quaternary ammonium compounds, make sure that the area is clean and without any soap residues to ensure effectivity. Avoid contact of the substance with eyes or hands. As above, wear gloves, suitable protective clothing, and eye or face protection to ensure safety. Keep area away from small children or pets to avoid any untoward incidents and in any case, do not swallow.

To use, dilute 25-50mL per 1 liter of water. As with bleach, you may put the solution in a bottle and spray the area to kill mold. Leave it on for a while then wipe or rinse off with potable water. Some quaternary ammonium compounds have pleasing smells and the fumes may not be an issue.

Compared to bleach, quaternary ammonium compounds are effective for longer and are more user-friendly. They also do not have an adverse effect on the surface used upon. There are no discolorations and smears. However, the product may be slower-acting than bleach.

Quaternary ammonium compound-based products have also been formulated with the sole purpose to kill mold compared to bleach-based products. There are claims that bleach or sodium hypochlorite do not actually kill mold and that quats are more effective in getting rid of mold. Bleach is still the faster way to kill mold.

Both bleach and quaternary ammonium compounds are advisable for killing mold on non-porous surfaces only, although most bathrooms are build with non-porous materials and it may not be a problem.

No matter which method you choose in killing mold, remember to exercise precautions and to choose the right and more effective product.


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