diff --git a/tox.ini b/tox.ini
index f1198620..34dbfa2e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,6 +11,8 @@ description = run tests
 setenv =
     devdeps: PIP_EXTRA_INDEX_URL =  https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
 
+passenv = OMP_NUM_THREADS
+
 deps =
     # We use these files to specify all the dependencies, and below we override
     # versions for specific testing schenarios