diff --git a/swh/loader/mercurial/identify.py b/swh/loader/mercurial/identify.py
--- a/swh/loader/mercurial/identify.py
+++ b/swh/loader/mercurial/identify.py
@@ -1,11 +1,10 @@
-# Copyright (C) 2020  The Software Heritage developers
+# Copyright (C) 2020-2021  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 from codecs import escape_decode  # type: ignore
 import json
-import os
 from pathlib import Path
 import re
 import subprocess
@@ -15,6 +14,7 @@
 # control
 import click
 
+from swh.loader.mercurial.utils import get_minimum_env
 from swh.model.cli import identify_object
 from swh.model.hashutil import hash_to_bytehex
 from swh.model.identifiers import normalize_timestamp, swhid
@@ -198,7 +198,7 @@
     def _output(self, *args) -> bytes:
         """Return the outpout of a `hg` call."""
         return subprocess.check_output(
-            ["hg", *args], cwd=self._root, env=self._get_env()
+            ["hg", *args], cwd=self._root, env=get_minimum_env()
         )
 
     def _call(self, *args) -> None:
@@ -208,18 +208,9 @@
             cwd=self._root,
             stderr=subprocess.PIPE,
             stdout=subprocess.PIPE,
-            env=self._get_env(),
+            env=get_minimum_env(),
         )
 
-    def _get_env(self) -> Dict[str, str]:
-        """Return the smallest viable environment for `hg` suprocesses"""
-        env = {
-            "PATH": os.environ["PATH"],
-            "HGPLAIN": "",  # Tells Mercurial to disable output customization
-            "HGRCPATH": "",  # Tells Mercurial to ignore config files
-        }
-        return env
-
     def root(self) -> Path:
         """Return the root of the Mercurial repository."""
         return self._root
diff --git a/swh/loader/mercurial/loader.py b/swh/loader/mercurial/loader.py
--- a/swh/loader/mercurial/loader.py
+++ b/swh/loader/mercurial/loader.py
@@ -34,6 +34,7 @@
 from swh.loader.core.loader import DVCSLoader
 from swh.loader.core.utils import clean_dangling_folders
 from swh.loader.exception import NotFound
+from swh.loader.mercurial.utils import get_minimum_env
 from swh.model import identifiers
 from swh.model.hashutil import (
     DEFAULT_ALGORITHMS,
@@ -136,6 +137,9 @@
         self.heads: Dict[bytes, Any] = {}
         self.releases: Dict[bytes, Any] = {}
         self.last_snapshot_id: Optional[bytes] = None
+        self.old_environ = os.environ.copy()
+        os.environ.clear()
+        os.environ.update(get_minimum_env())
 
     def pre_cleanup(self):
         """Cleanup potential dangling files from prior runs (e.g. OOM killed
@@ -152,6 +156,8 @@
         """Clean temporary working directory
 
         """
+        os.environ.clear()
+        os.environ.update(self.old_environ)
         if self.bundle_path and os.path.exists(self.bundle_path):
             self.log.debug("Cleanup up working bundle %s" % self.bundle_path)
             os.unlink(self.bundle_path)
diff --git a/swh/loader/mercurial/utils.py b/swh/loader/mercurial/utils.py
--- a/swh/loader/mercurial/utils.py
+++ b/swh/loader/mercurial/utils.py
@@ -4,7 +4,8 @@
 # See top-level LICENSE file for more information
 
 from datetime import datetime, timezone
-from typing import Optional, Union
+import os
+from typing import Dict, Optional, Union
 
 from dateutil.parser import parse
 
@@ -27,3 +28,17 @@
         return parse(visit_date)
 
     raise ValueError(f"invalid visit date {visit_date!r}")
+
+
+def get_minimum_env() -> Dict[str, str]:
+    """Return the smallest viable environment for `hg` suprocesses"""
+    env = {
+        "HGPLAIN": "",  # Tells Mercurial to disable output customization
+        "HGRCPATH": "",  # Tells Mercurial to ignore config files
+    }
+    path = os.environ.get("PATH")
+    if path:
+        # Sometimes (in tests for example), there is no PATH. An empty PATH could be
+        # interpreted differently than a lack of PATH by some programs.
+        env["PATH"] = path
+    return env