diff --git a/README.md b/README.md
index a33dad4..990005f 100644
--- a/README.md
+++ b/README.md
@@ -34,6 +34,17 @@ Usage
     -- It is also possible to use the env option to add additional variables to the environment
     sandbox('return foo', {env = {foo = 'This was on the environment'}})
 
+    -- The variables defined on the env are deep-copied and changes on them will not be persisted
+    local env = {foo = "can't touch this"}
+    sandbox('foo = "bar"', {env = env})
+    assert(env.foo = "can't touch this")
+
+    -- If you want to modify variables from inside the sandbox, use the refs option:
+    local refs = {foo = "kindof insecure"}
+    sandbox('foo = "changed"', {refs = refs})
+    assert(refs.foo = "changed")
+
+
 Installation
 ============
 
diff --git a/sandbox.lua b/sandbox.lua
index 034f4eb..696acc1 100644
--- a/sandbox.lua
+++ b/sandbox.lua
@@ -67,9 +67,10 @@ copy = function(other)
 end
 
 
-local function cleanup()
+local function cleanup(sandboxed_env, refs)
   debug.sethook()
   string.rep = string_rep
+  for k,_ in pairs(refs) do refs[k] = sandboxed_env[k] end
 end
 
 local function run(f, options)
@@ -79,8 +80,10 @@ local function run(f, options)
 
   local quota = options.quota or 500000
   local env   = options.env   or {}
+  local refs  = options.refs  or {}
 
   local sandboxed_env = merge(copy(BASE_ENV), env)
+  for k,v in pairs(refs) do sandboxed_env[k] = v end
 
   setfenv(f, sandboxed_env)
 
@@ -93,7 +96,7 @@ local function run(f, options)
   local timeout = function(str)
     instructions_count = instructions_count + 1
     if instructions_count >= quota then
-      cleanup()
+      cleanup(sandboxed_env, refs)
       error('Quota exceeded: ' .. tostring(instructions_count) .. '/' .. tostring(quota) .. ' instructions')
     end
   end
@@ -102,7 +105,7 @@ local function run(f, options)
 
   local ok, result = pcall(f)
 
-  cleanup()
+  cleanup(sandboxed_env, refs)
 
   if not ok then error(result) end
   return result
diff --git a/spec/sandbox_spec.lua b/spec/sandbox_spec.lua
index da406d5..df3e539 100644
--- a/spec/sandbox_spec.lua
+++ b/spec/sandbox_spec.lua
@@ -58,19 +58,49 @@ describe('sandbox', function()
 
     it('accepts a quota param', function()
       assert.no_has_error(function() sandbox("for i=1,100 do end") end)
-      assert.has_error(function() sandbox("for i=1,100 do end", {quota = 50}) end)
+      assert.has_error(function() sandbox("for i=1,100 do end", {quota = 20}) end)
     end)
 
   end)
 
-  describe('when given an env', function()
+  describe('when given an env option', function()
     it('is available on the sandboxed env', function()
       assert.equal(1, sandbox("return foo", {env = {foo = 1}}))
     end)
 
-    it('does not hide previous env', function()
+    it('does not hide base env', function()
       assert.equal('HELLO', sandbox("return string.upper(foo)", {env = {foo = 'hello'}}))
     end)
+
+    it('can not modify the env', function()
+      local env = {foo = 1}
+      sandbox("foo = 2", {env = env})
+      assert.equal(env.foo, 1)
+    end)
+  end)
+
+  describe('when given a refs option', function()
+    it('is available on the sandboxed env', function()
+      assert.equal(1, sandbox("return foo", {refs = {foo = 1}}))
+    end)
+
+    it('does not hide base env', function()
+      assert.equal('HELLO', sandbox("return string.upper(foo)", {refs = {foo = 'hello'}}))
+    end)
+
+    it('can modify the refs', function()
+      local refs = {foo = 1}
+      sandbox("foo = 2", {refs = refs})
+      assert.equal(refs.foo, 2)
+    end)
+
+    it('can modify the ref tables keys', function()
+      local refs = {items = {quantity = 1}}
+      sandbox("items.quantity = 2", {refs = refs})
+      assert.equal(refs.items.quantity, 2)
+    end)
+
+
   end)