Opinion by JUDGE HUME

In this workers' compensation proceeding, John Waddell (claimant) seeks review of the order of the Industrial Claim Appeals Office (Panel) which granted the claim of Eugene and Francis Haskin, doing business as Morning Star Arabians (employer), and their insurer, Colorado Compensation Insurance Authority (CCIA), for apportionment of liability. We affirm.

Claimant sustained an industrial injury to his back in 1992 while working for employer. Prior to that injury, claimant had suffered knee and ear injuries while in the military from 1979 to 1984 and had incurred another work-related injury in 1991, also while working for employer. The Administrative Law Judge (ALJ) found that claimant was permanently and totally disabled as a result of the combined effect of all of his injuries. Additionally, the ALJ attributed ten percent of claimant's disability to the military injuries, sixty percent to the 1991 injury, and thirty percent to the 1992 injury. Because the military injuries were non-industrial, the ALJ denied an offset to the Subsequent Injury Fund (SIF) and found employer and CCIA liable for 90% of claimant's permanent total disability (PTD) compensation.

On review, the Panel upheld the apportionment, but determined that employer and CCIA were liable for only 30% of the award.

I.

Claimant first contends that the Panel erred when it determined that the SIF was not responsible for any part of the award. We disagree.

Injuries incurred during active military service are not considered industrial injuries within the meaning of 8-46-101, C.R.S. 1997, the statute governing compensability under the SIF. City County of Denver v. Industrial Claim Appeals Office, 892 P.2d 429 (Colo.App. 1994). Consequently, the SIF is not liable, when, as here, a claimant's prior military-related injuries contribute to his total disability. City County of Denver v. Industrial Commission, 690 P.2d 199 (Colo. 1984) (SIF is not liable where the claimant's PTD is not caused exclusively by industrial disabilities).

Despite claimant's urgings to the contrary, we agree with the holding that injuries during military service are not industrial in nature and are bound by the precedential ruling regarding the scope of the SIF's liability.

II.

In view of our holding that there is no SIF liability, claimant, alternatively, asserts that the Panel erred in finding employer liable for less than his full disability. Again, we disagree.

Section 8-42-104(2), C.R.S. 1997, which governs apportionment, provides that:

In case there is a previous disability, the percentage of disability for a subsequent injury shall be determined by computing the percentage of the entire disability and deducting therefrom the percentage of the previous disability as it existed at the time of the subsequent injury. In such cases awards shall be based on said computed percentage. Such computation, when applicable, shall be made in the following types of awards under articles 40 to 47 of this title: Permanent total, permanent partial, including scheduled, working unit, and lump sum; except that, in the event the provisions of section 8-46-101 are applicable, such apportionment shall not be made.

A.

Claimant initially argues that the employer should be held liable for the entire disability under the "full responsibility" rule. We are not persuaded.

Pursuant to the full responsibility rule, an employer who hires a partially disabled employee is required to pay the entire disability award if the worker suffers another industrial injury and is declared permanently and totally disabled as a result. Subsequent Injury Fund v. Thompson, 793 P.2d 576 (Colo. 1990). The rule operates in the absence of a statute authorizing apportionment. Colorado Fuel Iron Corp. v. Industrial Commission, 151 Colo. 18, 379 P.2d 153 (1962).

Relying on City County of Denver v. Industrial Commission, supra, claimant asserts that the full responsibility rule takes effect when no liability exists for the SIF. However, that case addressed only the question of SIF liability for non-industrial injuries, and not the application of 8-42-104(2).

Further, in Askew v. Industrial Claim Appeals Office, 927 P.2d 1333 (Colo. 1996) (fn. 10), the supreme court recognized that the announcement of the full responsibility rule in Colorado Fuel Iron Corp. v. Industrial Commission, supra, occurred prior to the existence of 8-42-104(2), and does not operate to bar application of that statute.

Accordingly, because there was no SIF liability, 8-42-104(2) was properly applied. See Colorado Fuel Iron Corp. v. Rhodes, 166 Colo. 82, 441 P.2d 652 (1968) (upholding apportionment of successive industrial injuries incurred during employment with the same employer).

B.

Claimant also contends that the apportionment of his disability under 8-42-104(2) violates the equal protection guarantees of the Fourteenth Amendment and Colo. Const. art. II, 25. We disagree.

When subjected to a constitutional attack, a statute is presumed to be constitutional, and the party challenging the statute bears the burden of proving it to be unconstitutional. Firelock Inc. v. District Court, 776 P.2d 1090 (Colo. 1989); Naiden v. Epps, 867 P.2d 215 (Colo.App. 1993).

Because fundamental rights are not implicated by the Workers' Compensation Act, 8-40-101, et seq., C.R.S. 1997, the rational basis test applies. Industrial Claim Appeals Office v. Romero, 912 P.2d 62 (Colo. 1996).

Under that standard, a statutory classification that singles out a group of persons for disparate treatment must be rationally based on differences that are real and not illusory, and must be reasonably related to a legitimate state interest. Culver v. Ace Electric, 952 P.2d 1200 (Colo.App. No. 96CA1759, September 18, 1997). If there exists any conceivable set of facts indicating that the classification serves a legitimate governmental purpose, the statute must be upheld against a facial attack regardless of whether its application may have a harsh impact. Pace Membership Warehouse v. Axelson, 938 P.2d 504 (Colo. 1997).

Claimant argues that he is being denied benefits while an employee who becomes permanently and totally disabled as the result of one industrial accident receives a complete award of benefits. Because both the claimant and the hypothetical employee have sustained permanent and total disability, but only the hypothetical employee can recover full benefits, we conclude that disparate treatment has occurred. However, because we further conclude that such disparate treatment fulfills certain legitimate government interests, we find no denial of equal protection. See Duran v. Industrial Claim Appeals Office, 883 P.2d 477 (Colo. 1994).

Apportionment addresses the problem that arises in instances in which the combined effect of successive injuries is far greater than would be reflected merely by adding the impairments for each separate injury. See 5 Larson's Workers' Compensation Law 59.10 (1997). Apportionment ensures that the employer pays for only that portion of the employee's total disability that can be attributed to the subsequent injury, and therefore, it ameliorates the severe effect the full responsibility rule has on employers. City County of Denver v. Industrial Commission, supra. Consequently, apportionment represents an accommodation of the stated legislative purposes of providing the quick and efficient delivery of benefits to injured workers at reasonable costs to employers. Section 8-40-102(1), C.R.S. 1997; Duran v. Industrial Claim Appeals Office, supra.

Further, by limiting an employer's liability for compensation, apportionment promotes the additional statutory goal of encouraging the return of partially disabled workers to the work force. See Climax Molybdenum Co. v. Walter, 812 P.2d 1168 (Colo. 1991); Electron Corp. v. Industrial Claim Appeals Office, 833 P.2d 821 (Colo.App. 1992).

Thus, given the legitimate governmental purposes served by apportionment, the disparate treatment suffered by claimant or others situated similarly to him does not constitute a denial of the right to equal protection.

Finally, inasmuch as claimant does not contest the finding that he suffered from preexisting disabilities prior to the 1992 injury or the percentages assigned to those prior injuries, we conclude it is unnecessary to remand this matter.

The order is affirmed.

JUDGE JONES concurs.

JUDGE BRIGGS specially concurs.