The union that represents professional firefighters in Fairfax County claimed yesterday there is a "very critical and dangerous shortage of manpower" in the county fire department and asked for the hiring of 150 additional firemen.

County Fire Chief George H. Alexander later agreed that the firefighter had a legistimate complain and that there are many days when not enough firemen are on duty to "appropriately" do their job.

"We are talking about deaths, deaths either to us or to the people of the county," said Glenn Benarick, a county firemen and an executive of Local 2068 of the International Association of Fire Fighters.

The union released figures showing an increases in fire losses from $5.7 million in 1976 to $83 million in 1977 and increase in fire-related deaths from 13 in 1976 to 28 in 1977.

Fairfax County Executive Leonard Whorton, who has proposed adding 59 new fire department jobs for the up-coming year, responded to the firemen's demands by saving that he felt safe from fire in the county and he thinks the department provides an "effective level of manpower to protect life and property."

Whorton said the proposed construction of three new fire stations in county, which will be manned by 58 of the 59 new firemen,will help fire protection. Whorton pointed out that 14 of the 28 fire-related deaths last year were caused in highway accidents rather than home fires.

The county now has just over 600 paid firefighters 139 of whom are on duty at any one time, according to department rules. The fireman's union said yesterday that a minimum of 245 paid fireman should be on duty at all time to adequately available fire equipment.

In addition, there are about 600 volunteer fire-fighters; the professionals estimate that 43 of the volunteers are available to fight fires at any one time.

Firemen yesterday could give no example of a county fire where lack of manpower led to increased property loss or death, but they warned that minimum staffing at county stations is spreading fire protection dangerously thin and that "sooner or later somebody is going to have to pay."

The county fire department estimated recently that, according to established national fire prevention standards, 254 additional firemen are needed to maintain equipment, a department spokesman said yesterday. Chief Alexander earlier this year submitted a request for 113 new personnel to Whorton for the upcoming fiscal year.

Fairfax County is the only major jurisdiction in the Washington area that does not pay firemen overtime in order to maintain a minimum number of firemen on duty at all times.

Fireman's union president Randy Fulford called the volunteers used to supplement the paid firefighters "unreliable" and said county residents "cannot count on them when they are needed." Chief Alexandra said yesterday the volunteers were generally reliable, but that their availability was often limited, for example, during the day, when most of them work.

"We are not like Chicago or a Philadelphia where they get one call right back smack on another one," Alexandra said. But he said that any fire that requires, due to lack of manpower at one station, the participation of other fire stations creates "a tremendous void in protection."

The problem of manpower shortages is compounded, Alexander said, by the amount of vacation and leave accumulated by the county's veteran fire fighting staff. The department is short about 83 men each day because of vacation and leave, Alexander said.