diff --git a/adafruit_pm25/__init__.py b/adafruit_pm25/__init__.py
index 45a3593..4795e52 100644
--- a/adafruit_pm25/__init__.py
+++ b/adafruit_pm25/__init__.py
@@ -74,7 +74,13 @@ def _read_into_buffer(self) -> None:
 
     def read(self) -> dict:
         """Read any available data from the air quality sensor and
-        return a dictionary with available particulate/quality data"""
+        return a dictionary with available particulate/quality data
+
+        Note that "standard" concentrations are those when corrected to
+        standard atmospheric conditions (288.15 K, 1013.25 hPa), and
+        "environmental" concentrations are those measure in the current
+        atmospheric conditions.
+        """
         self._read_into_buffer()
         # print([hex(i) for i in self._buffer])