diff --git a/log.cpp b/log.cpp
index ab17e01b..77b61470 100644
--- a/log.cpp
+++ b/log.cpp
@@ -54,6 +54,9 @@ ALL_LEVELS[] = {
 static util::level_t
 to_level (std::string name)
 {
+    if (std::empty (name))
+        return util::EMERGENCY;
+
     static const std::map<std::string, util::level_t> NAME_LEVELS = {
         { "EMERGENCY",     util::EMERGENCY },
         { "ALERT",         util::ALERT },
@@ -106,8 +109,14 @@ util::operator<< (std::ostream& os, util::level_t l)
 
 
 ///////////////////////////////////////////////////////////////////////////////
-static util::level_t
-log_level (void)
+// Determine what the value for LOG_LEVEL should be at the beginning of
+// execution given the system environment.
+//
+// Note that the LOG macros _cannot_ be used from within this function as it
+// will likely result in infinite recursion.
+static
+util::level_t
+initial_log_level (void)
 {
     const char *env = getenv ("LOG_LEVEL");
     if (!env)
@@ -116,12 +125,42 @@ log_level (void)
     try {
         return to_level (env);
     } catch (...) {
-        LOG_ERROR("Invalid environment LOG_LEVEL: '%s'", env);
+        std::clog << "Invalid environment LOG_LEVEL: '" << env << "'\n";
         return util::DEFAULT_LOG_LEVEL;
     }
 }
 
 
+//-----------------------------------------------------------------------------
+// We shouldn't ever actually get to use the default value, but we set it to
+// the most verbose option just in case we've made a mistake elsewhere.
+
+static bool          s_log_level_done;
+static util::level_t s_log_level_value;
+
+//-----------------------------------------------------------------------------
+util::level_t
+util::log_level (level_t _level)
+{
+    s_log_level_value = _level;
+    s_log_level_done  = true;
+    return s_log_level_value;
+}
+
+
+//-----------------------------------------------------------------------------
+util::level_t
+util::log_level (void)
+{
+    if (!s_log_level_done) {
+        s_log_level_value = initial_log_level ();
+        s_log_level_done  = true;
+    }
+
+    return s_log_level_value;
+}
+
+
 //-----------------------------------------------------------------------------
 static bool
 needs_break (util::level_t level)
@@ -195,8 +234,7 @@ level_width (void)
 void
 util::log (util::level_t level, const std::string &msg)
 {
-    static const util::level_t LOG_LEVEL = log_level ();
-    if (level <= LOG_LEVEL) {
+    if (level <= log_level ()) {
         static const size_t time_len = strlen("YYYY-mm-dd HHMMhSS") + 1;
         std::string time_string (time_len - 1, '\0');
         time_t unix_time = time (nullptr);
@@ -209,7 +247,7 @@ util::log (util::level_t level, const std::string &msg)
             return;
         }
 
-        std::cerr << time_string << " ["
+        std::clog << time_string << " ["
             << level_colour (level)
             << std::setw (trunc_cast<int> (level_width ()))
             << std::left
diff --git a/log.hpp b/log.hpp
index c9e3ee1c..aa30788c 100644
--- a/log.hpp
+++ b/log.hpp
@@ -32,7 +32,6 @@ namespace util {
     ///////////////////////////////////////////////////////////////////////////
     // rfc5424 log levels. It is assumed they are contiguous to simplify array
     // indexing in logging code.
-    //
     enum level_t {
         EMERGENCY,              /** system is unusable */
         ALERT,                  /** action must be taken immediately */
@@ -58,14 +57,39 @@ namespace util {
     operator<< (std::ostream&, level_t);
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    // Query or set the global logging level filter. If the severity passed to
+    // a logging call is less than this then the output is discarded.
+    //
+    // The initial logging level can be controlled by setting the environment
+    // variable LOG_LEVEL to a string corresponding to the names of the values
+    // in level_t.
+    //
+    // Otherwise we fall back to the hard coded default from above.
+    //
+    // As a special case, a blank value for the environment variable LOG_LEVEL
+    // corresponds to the maximum log level. It indicates a preference for
+    // minimum output, and may be changed to disable all output in future
+    // updates.
+    level_t log_level (void);
+    level_t log_level (level_t);
+
     ///////////////////////////////////////////////////////////////////////////
     void log (level_t, const std::string &msg);
 
+
     template <typename ...Args, size_t N>
-    void log (level_t, const char (&fmt)[N], const Args&...);
+    void
+    log (level_t, const char (&fmt)[N], const Args&...);
 
 
     //-------------------------------------------------------------------------
+    // Various convenience macros for logging specific strings with a well
+    // known severity.
+    //
+    // LOG_DEBUG is treated similarly to assert; if NDEBUG is defined then we
+    // compile out the statement so as to gain a little runtime efficiency
+    // speed.
     #define LOG_EMERGENCY(...)  do { util::log(util::EMERGENCY, ##__VA_ARGS__); } while (0)
     #define LOG_ALERT(...)      do { util::log(util::ALERT,     ##__VA_ARGS__); } while (0)
     #define LOG_CRITICAL(...)   do { util::log(util::CRITICAL,  ##__VA_ARGS__); } while (0)
@@ -74,7 +98,7 @@ namespace util {
     #define LOG_WARN(...)       do { util::log(util::WARN,      ##__VA_ARGS__); } while (0)
     #define LOG_NOTICE(...)     do { util::log(util::NOTICE,    ##__VA_ARGS__); } while (0)
     #define LOG_INFO(...)       do { util::log(util::INFO,      ##__VA_ARGS__); } while (0)
-#if defined(ENABLE_DEBUGGING)
+#if !defined(NDEBUG)
     #define LOG_DEBUG(...)      do { util::log(util::DEBUG,     ##__VA_ARGS__); } while (0)
 #else
     #define LOG_DEBUG(...)      do { ; } while (0)