Radiation Map: How to Read US Radiation Monitoring

Radiation maps are easy to find and easy to misread. Here is where to look and what the numbers do and do not tell you.

Start with EPA RadNet

EPA's RadNet is the official US network. EPA says it has 140 radiation air monitors in 50 states, running around the clock and collecting near-real-time measurements of gamma radiation. Its public dashboard shows graphs for a specific location. EPA says the data are normally posted six times a day, so a graph can be hours behind what is happening now.

How to read a RadNet graph

• Count rate is not a dose. EPA says the gamma gross count rate shows how many gamma rays a monitor detects each minute from particles collected on its air filter, and that there is no direct relation between gamma gross count rate and exposure rate. Most monitors also report exposure rate, which is a separate measurement of radiation near the monitor.
• On a count-rate graph, look at change, not the number. EPA uses results over time to learn what is normal at each location and to spot unusual changes. Compare a station to its own history, not to another city.
• Normal levels vary. EPA says background depends on altitude, naturally occurring radioactive materials in the soil and air, and weather. Rain, snow and pressure changes can move readings up or down.
• A gap does not mean no radiation. EPA lists monitor maintenance or repair and server or network problems as causes of delays.

Fluctuations at one station are common, and EPA says trained scientists review flagged readings. A change on a graph is a reason to look at official sources. It is not an instruction to do anything by itself.

Units you will see

US maps often use roentgens or rem, and others use sieverts. EPA's conversions include 1 sievert = 100 rem and 1 gray = 100 rad. FEMA's planning thresholds use R/hr, so check the unit on any map before comparing numbers.

Volunteer maps

Community networks publish readings from privately owned detectors. They can fill gaps and show patterns, but instruments, placement and calibration differ, and we know of no agency that checks them. Treat a surprising reading as a reason to look at official sources, not as a fact.

In a real emergency

A map does not replace instructions. Ready.gov says to get inside, stay inside and stay tuned to officials. Federal planners warn that fallout levels change quickly and can be lethal early on, so do not go outside to check, and do not make decisions from a volunteer map.

See what fallout maps can tell you, how home detectors work and US nuclear plants by state.