You are a contractor or plumber, and you have been called in by a homeowner or building manager who has discovered a roof, plumbing, or HVAC leak. If that leak has been happening for more than a few days, it is likely that mold will be growing on an exposed surface and/or inside a wall or ceiling. Are there reasons to be concerned? You bet.
First, there are personal health issues. Most people react to high concentrations of mold spores with hay fever type symptoms - cough, runny nose and eyes, headache, etc. Many react badly to the chemicals that are part of the 'musty smell'. If you have asthma, breathing either of these may trigger an attack. Some people have an allergic reaction to spores, depending in part on their previous exposure to mold.
A good reason to be cautious, even if you don't react much now, is that repeated exposures may build up sensitivity, so at some point in the future, you may have a severe reaction; or you may develop chronic health problems, such as fatigue or memory loss. Also, depending on the type of mold, there is danger from toxins that can cause serious illness of the liver or nervous system.
Another reason for concern is danger to others. Opening a wall or ceiling may lead to a sudden release of mold spores into the area in which people are living or working. You don't want to jeopardize their health, nor do you want to risk a lawsuit. And if there is mold present and you don't take the proper precautions, whatever you do to fix the leak may make the air worse and spread the mold around.
How do you know if there is a potential problem? That 'musty odor' is one clue. If you open a wall and see mold growing on the surfaces inside, you can be sure. You may have heard about 'black mold' or 'toxic mold' - it's been in all the newspapers - and indeed some molds are worse than others, but mold of any color and type can be dangerous.
What should you do? To be truly safe, you should be wearing a good respirator and other protection anytime you open up a wall or go into a poorly ventilated space. After all, you're there because of a leak, and water damage almost always results in mold. If you see mold inside a wall, before you decide what to do next, contain it by sealing plastic over the opening. If mold is growing on an exposed surface, don't disturb it, as this would send spores into the air.
To know what you are dealing with - Is it especially toxic mold? How widespread is it? Can the process of fixing the leak make mold exposure worse? Should I recommend that a mold remediator be called before proceeding with repair? - it may be wise to call in a professional mold investigator to answer these questions.