Radiation Exposure – How much is too much?

A lovely quartzite slab being tested.

Radiation Exposure

How Much Is “Too Much”

Alex Stadtner, president of Healthy Building Science, wrote a blog in April 2012 entitled “Radioactive Granite and Quartzite – Risks from Radon” about discovering high levels of radiation in Quartzite and Granite used in residential building for items such as countertops, flooring, and bath and shower enclosures.  While that blog explained very well the risks of radiation in some common natural stone, I wanted to go into more detail about measuring radiation, and what the numbers mean.

Radiation is ubiquitous.  The earth is constantly being bombarded with cosmic rays as well as other radiation (x-rays, gamma, alpha, and beta radiation).  The earth’s magnetic field does an admirable job of blocking a majority of this radiation from reaching the earth’s surface, but a small percentage makes it and sometimes we are exposed to it.  Flying at 30,000 feet in an airplane or living at high altitude can increase exposure.
Some radiation is man made, X-ray machines, MRI, Nuclear power plants, weapons, and nuclear waste for example.EPA Radiation From Granite

Some radiation comes from the natural decay of radioactive material found in the earth.  This decay creates Radon gas is some areas.  This is the radiation that can be emitted by natural stone installed in a home.

This is an excellent article from MIT about radiation exposure limits:

http://web.mit.edu/newsoffice/1994/safe-0105.html

Excerpt from the above referenced article:
“The average exposure in the United States, from natural sources of radiation (mostly cosmic radiation and radon), is 300 millirems per year at sea level. Radiation exposure is slightly higher at higher elevations-thus the exposure in Denver averages 400 millirems per year.”


The current federal occupational limit of exposure per year for an adult (the limit for a worker using radiation) is “as low as reasonably achievable; however, not to exceed 5,000 millirems” above the 300+ millirems of natural sources of radiation and any medical radiation.

Minor: 500 Millirems

The maximum permissible exposure for a person under 18 working with radiation is one-tenth the adult limit or not to exceed 500 millirems per year above the 300+ millirems of natural sources, plus medical radiation. This was established in 1957 and reviewed as recently as 1990.

Fetus: 500 Millirems Or 50 Per Month (New Rule Jan. 1, 1994)

New federal regulations went into effect New Year’s Day, establishing for the first time an exposure limit for the embryo or fetus of a pregnant woman exposed to radiation at work. The limit for the gestation period is 500 millirems, with a recommendation that the exposure of a fetus be no more than 50 millirems per month.”

So there is not so much a “safe” level as much as a maximum lifetime exposure, with monthly and yearly recommendations to keep exposure below that level.  The goal is to have your background levels well below these recommendations.

Our instruments measure millirems/hour and we usually take a 15-minute sample.  300 millirems a year equates to 0.34 mR/Hr.  Today in the office I measured background at 0.014 mR/Hr.

For a fetus 50 millirems per month equates to 0.069 mR/Hr.Radioactivity Baseline

For measuring homes and stone products, I would consider any level above 0.018 mR/Hr to be a fail (this is equal to 13 millirems per month, about 25% of the recommended max exposure for a fetus).   Levels below 0.010 may be difficult to obtain due to natural background radiation levels.  As well, any reading that is more than 150% of the background control reading is also a fail.

It is a good idea to measure your home inside and out to determine background levels, or to discover any currently installed sources prior to any renovation.  Peace of mind, or discovering a hidden source of radiation, can be the result of testing an existing home.  Basements and crawlspaces, as well as occupied spaces should be tested for Radon gas being emitted from the soil.

I would recommend testing any new natural stone products prior to purchasing and installing them.

If we are measuring natural stone, it is best to compare to the background level (a sample taken in the parking lot at least 100 feet away from any stone products) and compare to the desired stone sample.

Radiation is everywhere, and invisible and asymptomatic, but can damage cells and in the worst-case lead to cancer.  Since we all spend so much time on our homes, knowing the level of radiation in your home and being emitted by the items installed in the home, is knowledge worth having.

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