diff --git a/lib/ratelimit.rb b/lib/ratelimit.rb
index 0d4efa8..378b7e2 100644
--- a/lib/ratelimit.rb
+++ b/lib/ratelimit.rb
@@ -40,7 +40,7 @@ def initialize(key, options = {})
   # @return [Integer] The counter value
   def add(subject, count = 1)
     bucket = get_bucket
-    subject = "#{@key}:#{subject}"
+    subject = get_key_for_subject(subject)
     redis.multi do
       redis.hincrby(subject, bucket, count)
       redis.hdel(subject, (bucket + 1) % @bucket_count)
@@ -55,14 +55,8 @@ def add(subject, count = 1)
   # @param [Integer] interval How far back (in seconds) to retrieve activity.
   def count(subject, interval)
     bucket = get_bucket
-    interval = [[interval, @bucket_interval].max, @bucket_span].min
-    count = (interval / @bucket_interval).floor
-    subject = "#{@key}:#{subject}"
-
-    keys = (0..count - 1).map do |i|
-      (bucket - i) % @bucket_count
-    end
-    return redis.hmget(subject, *keys).inject(0) {|a, i| a + i.to_i}
+    keys = get_bucket_keys_for_interval(bucket, interval)
+    return redis.hmget(get_key_for_subject(subject), *keys).inject(0) {|a, i| a + i.to_i}
   end
 
   # Check if the rate limit has been exceeded.
@@ -108,12 +102,58 @@ def exec_within_threshold(subject, options = {}, &block)
     yield(self)
   end
 
+  # Execute a block and increment the count once the rate limit is within bounds.
+  # This fixes the concurrency issue found in exec_within_threshold
+  # *WARNING* This will block the current thread until the rate limit is within bounds.
+  #
+  # @param [String] subject Subject for this rate limit
+  # @param [Hash] options Options hash
+  # @option options [Integer] :interval How far back to retrieve activity.
+  # @option options [Integer] :threshold Maximum number of actions
+  # @option options [Integer] :increment
+  # @yield The block to be run
+  #
+  # @example Send an email as long as we haven't send 5 in the last 10 minutes
+  #   ratelimit.exec_with_threshold(email, [:threshold => 5, :interval => 600, :increment => 1]) do
+  #     send_another_email
+  #   end
+  def exec_and_increment_within_threshold(subject, options = {}, &block)
+    options[:threshold] ||= 30
+    options[:interval] ||= 30
+    options[:increment] ||= 1
+    until count_incremented_within_threshold(subject, options)
+      sleep @bucket_interval
+    end
+    yield(self)
+  end
+ 
   private
 
   def get_bucket(time = Time.now.to_i)
     ((time % @bucket_span) / @bucket_interval).floor
   end
 
+  def get_bucket_keys_for_interval(bucket, interval)
+    interval = [[interval, @bucket_interval].max, @bucket_span].min
+    count = (interval / @bucket_interval).floor
+    (0..count - 1).map do |i|
+      (bucket - i) % @bucket_count
+    end
+  end
+
+  def get_key_for_subject(subject)
+    "#{@key}:#{subject}"
+  end
+
+  def count_incremented_within_threshold(subject, options)
+    bucket = get_bucket
+    keys = get_bucket_keys_for_interval(bucket, options[:interval])
+    evalScript = 'local a=KEYS[1]local b=tonumber(ARGV[1])local c=tonumber(ARGV[b+2])local d=tonumber(ARGV[b+3])local e=tonumber(ARGV[b+4])local f=tonumber(ARGV[b+5])local g=tonumber(ARGV[b+6])local h=0;local i=false;for j,k in ipairs(redis.call("HMGET",a,unpack(ARGV,2,b+1)))do h=h+(tonumber(k)or 0)end;if h<f then redis.call("HINCRBY",a,c,g)redis.call("HDEL",a,(c+1)%d)redis.call("HDEL",a,(c+2)%d)redis.call("EXPIRE",a,e)i=true end;return i'
+    evalKeys = [get_key_for_subject(subject)]
+    evalArgs = [keys.length, *keys, bucket, @bucket_count, @bucket_expiry, options[:threshold], options[:increment]]
+    redis.eval(evalScript, evalKeys, evalArgs)
+  end
+
   def redis
     @redis ||= Redis::Namespace.new(:ratelimit, redis: @raw_redis || Redis.new)
   end
diff --git a/scripts/count_and_increment_within_threshold.lua b/scripts/count_and_increment_within_threshold.lua
new file mode 100644
index 0000000..c341ed2
--- /dev/null
+++ b/scripts/count_and_increment_within_threshold.lua
@@ -0,0 +1,21 @@
+local subject = KEYS[1];
+local numKeys = tonumber(ARGV[1]);
+local bucket = tonumber(ARGV[numKeys + 2]);
+local bucketCount = tonumber(ARGV[numKeys + 3]);
+local bucketExpiry = tonumber(ARGV[numKeys + 4]);
+local threshold = tonumber(ARGV[numKeys + 5]);
+local increment = tonumber(ARGV[numKeys + 6]);
+local count = 0;
+local success = false;
+
+for key, value in ipairs(redis.call("HMGET", subject, unpack(ARGV, 2, numKeys + 1))) do count = count + (tonumber(value) or 0) end;
+
+if count < threshold then
+  redis.call("HINCRBY", subject, bucket, increment);
+  redis.call("HDEL", subject, (bucket + 1) % bucketCount);
+  redis.call("HDEL", subject, (bucket + 2) % bucketCount);
+  redis.call("EXPIRE", subject, bucketExpiry);
+  success = true;
+end
+
+return success;
\ No newline at end of file
diff --git a/scripts/count_and_increment_within_threshold.min.lua b/scripts/count_and_increment_within_threshold.min.lua
new file mode 100644
index 0000000..49d81d5
--- /dev/null
+++ b/scripts/count_and_increment_within_threshold.min.lua
@@ -0,0 +1 @@
+local a=KEYS[1]local b=tonumber(ARGV[1])local c=tonumber(ARGV[b+2])local d=tonumber(ARGV[b+3])local e=tonumber(ARGV[b+4])local f=tonumber(ARGV[b+5])local g=tonumber(ARGV[b+6])local h=0;local i=false;for j,k in ipairs(redis.call("HMGET",a,unpack(ARGV,2,b+1)))do h=h+(tonumber(k)or 0)end;if h<f then redis.call("HINCRBY",a,c,g)redis.call("HDEL",a,(c+1)%d)redis.call("HDEL",a,(c+2)%d)redis.call("EXPIRE",a,e)i=true end;return i
\ No newline at end of file