Labor and services of workmen at manual and mechanical labor in the employment of a person in an occupation declared by section 23-803 to be hazardous is service in a hazardous occupation within the meaning of the terms of section 23-801. By reason of the nature and conditions of and the means used and provided for doing the work in a hazardous occupation, such service is especially dangerous and hazardous to the workmen because of risks and hazards inherent in such occupations and which are unavoidable by the workmen therein.