The United States Environmental Protection Agency (EPA) has a radiation monitoring website called RadNet. From the homepage:
"The nationwide RadNet system monitors the nation’s air, drinking water, precipitation, and pasteurized milk to determine levels of radiation in the environment. RadNet sample analyses and monitoring results provide baseline data on background levels of radiation in the environment and can detect increased radiation from radiological incidents."
From the RadNet FAQ:
"RadNet is the only nationwide system that continuously monitors of ambient environmental radiation levels and those resulting from major nuclear accidents, such as the Fukushima nuclear reactor incident in Japan. RadNet data are used to inform the public, providing assurance if contamination levels are very low or helping to make science-based decisions about taking protective actions if contamination levels are high enough to warrant them."
The EPA has stationary and mobile air monitors measuring radioactivity 24 hours a day, 7 days a week. Monitoring sites were chosen to give comprehensive coverage to the environment in the United States. These readings are consolidated and rendered as graphs several times a day.
The fixed air monitors pass about 60 cubic meters of air per hour through a filter which is constantly checked for beta and gamma radiation. An average person will breathe about 20 cubic meters of air per days.
There is a map of the air monitoring stations in the US displayed on the website.
You can put in your address to find a monitoring station near you. The locator includes a slider that allows you to enter the distance you want the search to encompass.
The system will zoom into the search zone and show monitoring stations as purple circles. Here is the search results for my zip code in Seattle, Washington.
Putting your cursor over the purple spot will show you where the monitor is.
Under the map is a list of states and cities in those states where you can check on the air measurements or the liquid measurements.
The air measurements show charts of gamma radiation:
The radiation measurements are for different ranges of gamma energies. The chart shows the different counts for emission events in these gamma ranges. The diagram below is an example of the total gamma count for a few hours for the ranges shown side by side.
The main purpose of the RadNet air monitoring is to establish a base line for normal background radiation across the United States and to monitor the air for significant departures from that normal level caused by detonations of nuclear bombs and incidents at nuclear reactors where radiation is released into the environment. The primary path of such radioactive pollution is through the atmosphere. Initial dangers to the health of U.S. citizens from heightened radioactivity is through inhalation of particles in the air.