Concise answer
A helmet can rub or compress healing recipient areas, so its use should restart only when the operating surgeon confirms the graft location and skin recovery can tolerate it.
Helmet fit, journey length, heat and sweating all affect friction and pressure rather than the calendar alone.
A loose cap is not automatically safe if it touches grafts or is removed carelessly.
When evaluation may help
Discuss transport needs before surgery and ask the clinic to review healing if helmet use is unavoidable for work or daily travel.