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Biocides (like chlorine bleach) are toxic to humans, as well as to mold.
- United States Environmental Protection Agency
Black molds are a group of ubiquitous fungi, belonging to the genus Stachybotrys, and are commonly seen growing on damp walls, ceilings, carpets and paper; as well as in toilets, showers, bathtubs, etc. The microscopic reproductive spores of these fungi are highly stable; and are present almost everywhere, be it soil, water or air.
Moisture is the most important factor that favors the germination and growth of such spores. Hence, excessive humidity and accumulation of moisture in walls, window panes, tiles and other building material, is often followed by the growth of molds.

Although indoor mold exposure does not pose severe health concerns, certain individuals are more sensitive to mold growth and may experience eye and skin irritation, nasal congestion, and sneezing. However, immunocompromised individuals may suffer severe health issues and respiratory disorders due to exposure to black mold.
Does Bleach Kill Black Mold?
black mold and bleach
► The common bleach or sodium hypochlorite (NaOCl), when dissolved in water dissociates into sodium ions (Na+), hypochlorous acid (HOCl) and hypochlorite ions (OCl-). Of these, HOCl and OCl- are strong oxidizing agents, and are responsible for inactivating black mold.
► The cell walls of black molds are made up of chitin and glucan, which can be easily penetrated by OCl- ions. These ions then disrupt the structural and functional integrity of their cell membranes. Once inside the cells, the chlorine ions combine with cellular components to form chloramines, which affect the normal cellular metabolism. This results in the death of mold cells.

► However, it is important remember that just killing black mold is not enough. The dead molds, spores and mold components are also capable of causing allergies and toxicity symptoms.
When to Use Bleach for Mold Removal?
► Although, OCl- ions can kill molds, these ions may lead to health concerns in humans as well. They may lead to allergic rashes, nausea, chest pain, or temporary difficulty in breathing in some individuals.

► Moreover, they are effective only when used for disinfecting non-porous or hard surfaces like shower panels, toilets, sinks, countertops, etc.

In case of porous surfaces like carpets, drywalls, etc., the use of bleach may seem temporarily useful but cannot get rid of the mold. This is simply because molds have specialized structures called mycelia, which can penetrate through the porous surfaces. In such cases, adding a bleach solution may inactivate the cells on the surface but not those inside the pores. On the contrary, water from the bleach solution may remain in the pores providing the cells with the most important life-supporting factor - moisture.
Agencies like EPA and CDC do not recommended the use of chlorine bleach to deal with molds; and state that it may be used only in specialized cases where residents are immunocompromised individuals. In such instances, the amount of bleach used should not be more than one cup for one gallon of water.
"Absorbent or porous materials like ceiling tiles, drywall, and carpet may have to be thrown away if they become moldy."
- Centers for Disease Control and Prevention (CDC)
Procedure for Black Mold Removal Using Bleach
► Prepare a bleach solution by adding ¾ cup of chlorine bleach to 1 gallon of water. Apply this solution onto the black mold growth spots present on the non-porous surface to be cleaned; and leave it untouched for 20 minutes at least.

► About 20-30 minutes of exposure to chlorine bleach is essential for the method to be effective. Rinse the bleach off thoroughly, and clean the surface with water. Dry the surface thoroughly.
Important Warning!
DO NOT mix bleach with household detergents!
Household detergents generally contain ammonia and acids, which may react with the bleach solution, and give rise to toxic fumes. Hence, always avoid mixing two or more cleaning solutions and detergents together.
Useful Tips
Given below are some precautionary measures as well as tips to be followed while using bleach to remove black molds.

~ Use appropriate eye wear, mask as well as non-porous gloves while handling bleach.
~ Ensure to keep the doors and windows open, so that the area is well ventilated while cleaning mold growth.
~ Always follow the instructions provided along with the bleach solution, and ensure that the concentration of the solution is not higher than the recommended value.
~ Do not use bleach to clean metal surfaces, and avoid its contact with wiring or HVAC (heating, ventilation, and air conditioning) systems.
~ In case of eye or skin irritation, immediately rinse the eyes or the affected skin part with water.
~ In the event of accidental ingestion of chlorine bleach, rinse your mouth with water, and seek medical help if nausea or vomiting is experienced.

The use of chlorine bleach and other biocides is effective only when the affected surfaces are nonporous; and should be used only when individuals with a weak immune system are residing in the room or building. Even in such cases, it is always advisable to seek professional help, especially if a large surface area has been affected by mold growth.

To avoid recurrent growth of black molds, maintain the rooms clean and dry by using proper ventilation systems and dehumidifiers. Give immediate attention to walls or surfaces which remain damp, or are damaged due to water leaks, and get the leakage repaired to prevent mold growth. Remember, moisture is the most vital growth factor for molds.