diff --git a/cddl/usr.bin/ctfmerge/ctfmerge.1 b/cddl/usr.bin/ctfmerge/ctfmerge.1
--- a/cddl/usr.bin/ctfmerge/ctfmerge.1
+++ b/cddl/usr.bin/ctfmerge/ctfmerge.1
@@ -1,5 +1,5 @@
 .\"
-.\" Copyright (c) 2010 The FreeBSD Foundation 
+.\" Copyright (c) 2010-2025 The FreeBSD Foundation 
 .\" All rights reserved. 
 .\" 
 .\" This software was developed by Rui Paulo under sponsorship from the
@@ -26,7 +26,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 
 .\" SUCH DAMAGE. 
 .\"
-.Dd April 21, 2022
+.Dd December 3, 2025
 .Dt CTFMERGE 1
 .Os
 .Sh NAME
@@ -104,6 +104,24 @@
 into
 .Ar destfile .
 .El
+.Sh ENVIRONMENT VARIABLES
+The following environment variables are used in
+.Nm :
+.Bl -tag -width x
+.It CTFMERGE_TERMINATE_NO_UNLINK
+Don't remove temporary files on exit, so that they can be examined
+for debugging.
+.It CTFMERGE_MAX_SLOTS
+Number of round-robin slots in the input queue that it is processed by
+worker threads.
+Each slot represents a file batch (default 5).
+.It CTFMERGE_PHASE1_BATCH_SIZE
+Number of files in a slot that are needed before processing starts (default 8).
+.It CTFMERGE_INPUT_THROTTLE
+Max queue size to queue (defaults to 10).
+.It CTFMERGE_DEBUG_LEVEL
+Increase debugging verbosity (maximum is 3).
+.El
 .Sh EXIT STATUS
 .Ex -std
 .Sh SEE ALSO