diff --git a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
index e7e1bb7..1a33550 100644
--- a/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
+++ b/ClosedXML/ClosedXML/ClosedXML/Extensions.cs
@@ -164,10 +164,27 @@
 
     public static class IntegerExtensions
     {
-       
+        private static readonly NumberFormatInfo nfi = CultureInfo.InvariantCulture.NumberFormat;
+        [ThreadStatic]
+        private static Dictionary<Int32, String> intToString;
         public static String ToInvariantString(this Int32 value)
         {
-            return value.ToString(CultureInfo.InvariantCulture);
+            String sValue;
+            if (intToString == null)
+            {
+                intToString = new Dictionary<int, string>();
+                sValue = value.ToString(nfi);
+                intToString.Add(value, sValue);
+            }
+            else
+            {
+                if (!intToString.TryGetValue(value, out sValue))
+                {
+                    sValue = value.ToString(nfi);
+                    intToString.Add(value, sValue);
+                }
+            }
+            return sValue;
         }
 
     }