What Causes Black Mold?

No one wants to find mold growing in their home.

Especially black mold.

Black mold is one of the worst feared types of mold and even if you don’t have an allergy to it, this mold can cause a number of health issues.

So, what causes black mold in the first place?

And why do so many homes have it?

That’s the question we’re going to answer in this post.

As you’ll discover, if there’s a good side at all to black mold, it’s that it requires some very specific conditions in order to grow.

What Causes Black Mold to Grow?

The specific conditions needed for black mold to grow indoors include the following:

  • Mold spores
  • A food source
  • The right place to grow
  • Oxygen
  • Moisture

Let’s look at each one of these in depth.

Mold Spores

All mold begins from spores.

These spores are incredibly small (microscopic), and as they move through the air, they settle onto ceilings, walls, and various other surfaces.

If the proper conditions are present, the spores will begin to grow into patches of black mold.

One of the easiest ways to prevent mold spores from floating around in your home is to use an air purifier.

Air purifiers that contain a specific set of air filtering technologies can trap and kill mold spores before they have a chance to grow into a harmful substance.

Check out our guide on the best air purifiers for mold here, to learn more about how they work to keep mold under control.

Food Source

When it comes to finding out what causes black mold, the thought of what it eats might not pass through your mind.

But black mold must have a food source to survive.

In general, this mold can eat practically anything that contains organic material, but it really enjoys items that contain high amounts of cellulose, with processed cellulose being the favorite.

Thus, paper products are black mold’s favorite food, but it can also thrive on:

  • Wood
  • Straw
  • Drywall
  • Particleboard
  • Cardboard
  • Paper of any type

And, in some cases, lint or dust that contains cellulose can create a sufficient food source for black mold to survive.

The Right Place to Grow

As long as the black mold has the right food source and the right amount of moisture, black mold can thrive just about anywhere.

This includes:

  • Clothes
  • Books
  • Walls
  • Ceilings
  • Furniture

Some of black mold’s favorite places to grow indoors include:

  • Attics
  • Basements
  • Crawl spaces

These locations are ideal because they aren’t cleaned as often and have everything that causes black mold to exist.


Mold needs oxygen to grow, but it doesn’t necessarily need it to survive.

If you think that by depriving black mold of this important element you can kill it, you would be wrong.

Mold has the ability to go into a dormant state for millions of years if oxygen isn’t present. Then, when the right conditions present themselves, they will come out of their “sleep” and start growing again.


One of the most important elements that black mold needs to grow is moisture.

In fact, all mold needs moisture to grow, but black mold needs more than other species.

The moisture can come from a variety of different sources, including:

  • Rain
  • Leaky plumbing
  • Humidity
  • Condensation

A simple way to keep areas like attics, basements, and crawl spaces from growing black mold is to use a dehumidifier to keep the moisture level low.

It’s also advised to fix any leaking plumbing or entry points around the house where rain seeps in.

The Top Conditions That Causes Black Mold

When it comes to what causes black mold, as long as black mold has the important elements listed above, it can grow just about anywhere.

However, for it to really thrive, there are some perfect conditions that are needed for this species, including:

  • Heat – Even though mold can survive in all temperatures, they grow better in warmer environments. Their ideal temperature is right around 73 degrees Fahrenheit, but it can exist in temperatures as high as 104 degrees Fahrenheit and as low as 36 degrees Fahrenheit. If the temperatures are outside of these ranges, the mold will go into a dormant state.
  • Humidity – Since black mold requires more moisture, it prefers environments that have a relative humidity of around 90%. However, once the growth process has started, it can thrive in areas that have 70% or more relative humidity.
  • Time – Black mold requires a bit more time and exposure to moisture before it gets going. Thus, it is common to find black mold in areas where flooding has occurred or if you have leaking pipes.
  • Minimal Light – Black mold can grow in complete darkness and might prefer those conditions at certain times. However, it can also thrive in low light conditions. It absolutely can’t grow in large quantities of light, and ultraviolet light kills black mold.

Common Causes of Black Mold Indoors

As you’ve learned, black mold needs very specific conditions to grow, and these can occur in any home.

This leaves you and anyone who enters your home at risk for this mold to have a negative health impact.

Here are the common reasons why black mold is caused and thrives indoors:

Water Leaks

If you have anything leaking in your home, including your air conditioner, plumbing or an outdoor faucet, this could create the perfect moisture conditions for black mold to grow.

Since many leaks go undetected for long amounts of time, this gives black mold exactly what it needs to become established.

High Humidity

Leaks are one of the main causes of black mold growth, but humidity can contribute as well.

Most modern homes are incredibly well sealed, as this makes them more efficient to heat and cool.

However, it also traps moisture in ceilings, walls, attics, and crawlspaces.

High levels of moisture combined with a dark space and warmth create the ideal conditions for black mold to grow.

How Long Does it Take Black Mold to Grow?

Most mold species will start growing within 24 to 48 hours after coming in contact with a water source.

However, for black mold, it can take up to 12 days for a patch to flourish.

But, it will still need the perfect environmental conditions to survive. Thus, if the water source dries out or too much light gets into the area, the black mold will have a hard time growing and may stop before it becomes a problem.

How to Prevent Black Mold from Growing Indoors

The best way to prevent mold growth in your home is to deny it the perfect environmental conditions.

This means getting rid of airborne mold spores, excess moisture, and warmth.

These preventative steps can be achieved by doing the following:

  • Add fans and ventilators where possible.
  • Use one of the best air purifiers for mold here to keep mold spores out of the air.
  • Use a dehumidifier to keep humidity levels low.
  • Call in a professional to check and repair any water leaks.

Summary of What Causes Black Mold

As you learned, black mold can have a negative impact on your health, and it’s one of the most feared molds in the world.

And the top reasons for what causes black mold are all conditions that you can control if you put in a little effort.

Make sure your home is unappealing to black mold by getting rid of the spores, excess moisture, humidity, and food sources.

By doing those things, you can live in a home without fear of a scary mold problem.