Rabbi Adin Steinsaltz writes:
By way of analogy, exposure to radiation above a certain intensity can prove fatal.
But also a lesser dose is destructive in the long term.
Although the person is not immediately killed, a steady process of deterioration begins, from the time of the exposure and onward.
Or, to cite another example, some poisons, such as lead, ingested even in the smallest of quantities, can never be removed.
Any immediate damage can usually be overcome, but the cumulative effect over time, as more and more of that material enters the body, can be extremely dangerous.
In the same way, every non-holy act, even if permitted, even if rectified and elevated to holiness, leaves a mark of profanity.
Thus, a person can degenerate spiritually merely by doing things that are permitted by The Code of Jewish Law.
The steady accumulation of mundane actions is liable to draw him out of the realm of holiness.
A decent person who never deliberately commits a sin, who merely allows himself a so-called normal life, can, in the half-hour or afternoon he devoted to activities devoid of holiness, create a mark of profanity in his body that cannot be removed.
–Rabbi Adin Steinsaltz