We think of granite as impervious to everything, and indeed, it is both hard and strong. Granite is a major part of the continental crust and is composed primarily of quartz, feldspar, mica, and horneblend. Chemically, it is approximately 72% SiO2 and 14% Al2O3 with other compounds comprising less than 5% each. Depending on the impurities present during formation, granite can be of many different colors.
Granite is very hard with a hardness rating between 6 and 7, with diamond as a reference of 10. It is also resistant to many chemicals and is even used in underwater applications. This makes it very useful as a counter top material. Normal cutlery used in homes and most food liquids will not harm granite provided they are cleaned soon after use.
Fungi are present on and even in granite in the natural environment. The fungal spores germinate on the surface of granite, and if water is present, the hyphae are able to force their way into the spaces between crystals using osmotic pressure. The fungi also release organic acids that can assist in penetration of the rock. This process occurs over long periods of time, and results from this are generally not relevant to granite counter tops in the home. Although water may be present, it is usually transient and there is no time for fungi to gain a foothold.
IN looking for cases with fungal growth, there are studies of bacteria sticking to granite materials, but no more so than to other kinds of counter top material. The two studies most often quoted are by Patricia Fajardo-Cavazos and Wayne Nicholson (Appl. Environ. Microbiol. 2006 April; 72(4): 2856-2863), and Berenice Thomason, James Biddle, and William Cherry (Appl. Microbiol., Nov. 1975, p. 764-767 Vol. 30, No. 5). The first of these studies was done by NASA and focused on extreme environments and the genus Bacillus, which makes extremely resistant endospores. They did find these spores in the natural environment, which is not surprising, but it does not extrapolate to any health risk in the home environment. The second study was done by the Centers for Disease Control, and examined the prevalence of Salmonella strains in natural environments. Again, they were found, but the data are not relevant to the home environment other than emphasizing that there are microorganisms everywhere except in places where they are specifically excluded.
As with any other surface used for handling food, scrupulous hygiene is important. The most important cause of food contamination is handling or preparing food for cooking on a surface such as chicken, then preparing uncooked food on the same surface without washing. Hydrogen peroxide is a good disinfectant as is very dilute bleach.
As with any other material, keeping granite dry is the best defense against the growth of mold and other microorganisms. Sealing the surface of polished granite can make it hydrophobic so that it is less likely to soak up any water. Even unsealed granite absorbs very little water. If the material remains wet for days at a time, surface mold growth will occur. Once the surface is dry, the mold can be removed using soap and water. You can also use proprietary granite cleaning solutions.
Granite contains 10-20 ppm uranium, which decays to produce radon gas. Although health physicists do agree that granite countertops may emit radon, the levels are insignificant compared with background levels.
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