Turnover among the nation’s chief executive officers fell 16.8 percent in June from the prior month, according to a report from Challenger, Gray & Christmas Inc.

The global outplacement consultancy firm said 107 departures were announced in June, marking the fifth consecutive month with more than 100 announced departures.

There were 125 CEO exits announced in May. In June 2009, the tally was 105.

Of the 673 CEO changes announced in the first six months of 2010 – an 11 percent increase from 607 during the same period last year – 201 CEOs cited resignation as their reason for departure, compared with 152 in the same period last year.

Retirement, the second most commonly cited reason for departure, led to 190 CEO exits so far this year, compared with 108 in the first half of 2009.

Another 112 CEOs stepped down from January to June, meaning they likely remained with their companies in some capacity.

Challenger said 19 CEOs were removed or pressured to leave by their companies’ governing boards, noting that is a sign of the volatile economy in which corporate leaders must operate.

Another 11 departing CEOs cited economic conditions for their exits in 2010, and four were ousted in the wake of some legal entanglement or other public scandal.

There were 34 involuntary departures in the first half of 2010, compared with 27 for the same period last year.

The sector experiencing the most turnover in the first half of 2010 was health-care, with 112 CEO exits, up from 95 a year ago.