Q:I live in San Diego, Calif., near the ocean. The house next door is under reconstruction. There has been a constant flow of red dust and sawdust, combined with several days of heavy fog. I now see that sections of our tile roof are now green with a type of mold that looks like cauliflower-shaped circles. I have since developed a very bad case of laryngitis because of a mold allergy. I am not intending to sue my neighbor. He is aware of the problem and wants to have the carpenters use a blower on our roof but I feel that will spread the spores. I think they should use some sort of solution that will kill the spores. What do you suggest?

A: Any time nutrients such as dust/sawdust, moisture and mold spores mix, fungal growth can occur. However, most molds are not green, particularly on the outside of a house. What you are seeing is likely to be algae, which is often green because it is photosynthetic. In a sunny spot, the ultraviolet rays inhibit fungal growth. The organic components of soil always have fungi or mold present (think decaying leaves). Microbial growth on the exterior of the building, no matter if it is mold or algae, may cause allergies or sensory irritation if an air intake or other pathway enters into the house. The red dirt and sawdust may have acted to absorb moisture and maintain its contact with the house's exterior envelope, thereby assisting microbial growth. Pollen and other airborne dust lands on our homes all the time, but heavy rains usually wash them away. I presume heavy rains are not common in San Diego.

The solution to preventing growth is to keep the impacted areas out of the shade and in heavy sunlight. Power-washing the building with a detergent (trisodium phosphate or TSP) will often remove any existing growth.

Another contributing factor may be the paint used in the construction of your house. In the past, house paint sometimes contained biocides to inhibit microbial growth. These biocides were made with toxic heavy metals but have been removed from today’s paints. Biocides can be purchased separately and added to paints and stains to inhibit microbial growth in the future.

Thanks to Mark Goldman, who is a senior air quality scientist for Engineering and Fire Investigations (EFI), a firm that provides mold investigation and indoor air quality services nationwide. Mark can be reached at mark_goldman@efiglobal.com

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