This case concerns the proper calculation of “average weekly wages” under G. L. c. 152, § 1 (1), where a union worker is injured on an entirely union public works project and entitled to workers’ compensation benefits. The reviewing board of the Department of Industrial Accidents (reviewing board) determined that employer payments into health and welfare plans, pension plans, and supplemental unemployment benefits (fringe benefits) must be included in determining the employee’s average weekly wages. Because we conclude that the plain language of the applicable statutes, G. L. c. 152, § 1 (1), as *362amended through St. 1991, c. 398, § 13, and G. L. c. 149, §§ 26 and 27, makes the inclusion of such employer payments mandatory, we affirm the reviewing board’s decision.
Facts and procedural history. This case has a long, complex procedural history which we need not belabor. Suffice it to say, William McCarty (employee), a member of Local 3 of the Bricklayer’s and Allied Craftsmen Union, worked for defendant Wilkinson & Company (employer) as a tile setter and grouter on Boston’s third harbor tunnel project (project). He was injured in 1994 and received workers’ compensation benefits from National Union Fire Insurance Company of Pittsburgh, Pennsylvania (insurer), until 2000. In March, 1996, an administrative judge concluded that because all the project’s employees were unionized, the computation of the employee’s average weekly wage should not include fringe benefits. On May 21, 1997, the reviewing board reversed the decision of the administrative judge, holding that the statutory scheme required the inclusion of fringe benefits. The case was remanded and the administrative judge amended his decision. Ultimately, the reviewing board affirmed his decision. The insurer and the employer appealed. We transferred their appeal from the Appeals Court pursuant to G. L. c. 211, § 4A.
Discussion. We begin by setting forth the relevant portions of the applicable statutes.
A permanently or partially incapacitated employee is entitled to receive compensation under the workers’ compensation statute. G. L. c. 152, §§ 34, 35. The amount is based on certain formulas in the statute. Id. Those formulas include a calculation of the employee’s average weekly wages.2 General Laws c. 152, § 1 (1), defines “average weekly wages” for injured employees. The second paragraph of that definition, inserted by St. 1991, c. 398, § 13, states, in pertinent part:
“Except as provided by sections twenty-six and twenty-seven of chapter one hundred forty-nine, such fringe benefits as health insurance plans, pensions, day care, or education and training programs provided by employer *363shall not be included in employee earnings for purposes of calculating the average weekly wages ...” (emphasis added).
General Laws c. 149, §§ 26 and 27, as amended through St. 1991, c. 552, § 94, and St. 1993, c. 110, § 173, respectively, referred to in the second paragraph of G. L. c. 152, § 1 (1), are part of the prevailing wage law, and allow the Commissioner of Labor and Industries to set wage rates on public works projects. Felix A. Marino Co. v. Commissioner of Labor & Indus., 426 Mass. 458, 459 (1998). Section 26 states, in relevant part:
“Payments by employers to health and welfare plans, pension plans and supplementary unemployment benefit plans under collective bargaining agreements or understandings between organized labor and employers shall be included for the purpose of establishing minimum wage rates as herein provided” (emphasis added).
Section 27 states, in relevant part:
“The aforesaid rates of wages in the schedule of wages shall include payments by employers to health and welfare plans, pension plans and supplementary unemployment benefit plans as provided in [§ 26], and such payments shall be considered as payments to persons under this section performing work as herein provided. Any employer engaged in the construction of such works who does not make payments to a health and welfare plan, a pension plan and a supplementary unemployment benefit plan, where such payments are included in said rates of wages, shall pay the amount of said payments directly to each employee engaged in said construction” (emphasis added).3
It is the language in these provisions on which the reviewing board relied in concluding that fringe benefits must be included in the determination of the employee’s average weekly wage.
The employer and the insurer make numerous arguments concerning the reviewing board’s interpretation of the exception *364contained in G. L. c. 152, § 1 (1), second par., including that, in light of the purpose of the 1991 amendments to G. L. c. 152; the plain language of the statutes; and the legislative history G. L. c. 149, §§ 26 and 27, the application of §§ 26 and 27 limited to those instances where the workplace is a mix union and nonunion employees and a nonunion employee is injured.4
“The workers’] compensation act is to be construed broadly, rather than narrowly, in the light of its purpose and, so far as reasonably may be, to promote the accomplishment of its beneficent design .... But it is also settled that, in construing a statute, its words must be given their plain and ordinary meaning according to the approved usage of language . . . and that the language of the statute is not to be enlarged or limited by construction unless its object and plain meaning require it.” Taylor’s Case, 44 Mass. App. Ct. 495, 499 (1998), quoting Johnson’s Case, 318 Mass. 741, 746-747 (1945). The language of G. L. c. 152, § 1 (1), second par., is clear and unambiguous. There is nothing in the plain language of the exception to the rule that fringe benefits are not part of the calculation of average weekly wages that indicates that it does not apply to entirely union public works projects, as the employer and insurer argue. The language clearly and simply refers to those sections of G. L. c. 149, §§26 and 27, that require that the calculation of “minimum wage rates” include fringe benefits.5 Had the Legislature intended to limit the exception to nonunion public works project employees, it could easily have done so. See Commissioner of Revenue v. Cargill, Inc., 429 Mass. 79, 82 (1999) (court may not amend statute’s language or infer legislative intention where language is clear).
Relying on Taylor’s Case, supra, the employer and the insurer argue that the purpose of the 1991 overhaul of the statute was to decrease the costs of workers’ compensation and, because the reviewing board’s construction of G. L. c. 152, § 1 (1), increases benefits to union workers on public works projects, *365the construction is erroneous. Even if the sole purpose of the 1991 amendments was to cut costs, the statutory scheme itself has a “beneficent design,” id. at 499, and is considered a “humanitarian measure.” CNA Ins. Cos. v. Sliski, 433 Mass. 491, 493 (2001), and cases cited. The employer and the insurer’s interpretation would have us infer an intention on the part of the Legislature beyond the plain language of the statute. See Commissioner of Revenue v. Cargill, Inc., supra. Moreover, St. 1991, c. 398, § 16, added the phrase “or decreases” to § 2A, so that it now reads: “Every act, in amendment of this chapter . . . which increases or decreases the amount or amounts of compensation payable to an injured employee or his dependents . . . .” Although the addition of the phrase “or decreases” indicates, as the employer and the insurer assert, that one of the purposes of the 1991 legislation was to decrease benefits to injured employees, Taylor’s Case, supra at 500-501, the Legislature did not strike the word “increases” from the statute, indicating at least that the Legislature recognized that some employees’ benefits would increase because of changes to the statute.6
In addition, the second paragraph reconciled the statute with the decisions of the reviewing board prior to 1991. Before the enactment of G. L. c. 152, § 1 (1) second par., in cases where employees were members of unions and working on public works projects, the reviewing board determined that fringe benefits were part of the calculation by relying, as it did in this case, on the provisions of G. L. c. 149, §§ 26 and 27. See, e.g., Lyons v. Fontaine Bros., 4 Mass. Workers’ Comp. Rep. 398, 399 (1990); Machado v. Joseph B. Fay Co., 3 Mass. Workers’ Comp. Rep. 38, 40 (1989). In the Machado decision, it is clear that the public works project had hired both union and nonunion employees. Id. at 39. Because the Lyons decision states that the facts “are virtually identical” to those in the Machado case, we assume that workplace also contained a similar mix of employees. Lyons v. Fontaine Bros., supra. In a decision that *366predates the Lyons decision, the reviewing board held that an employee who was not a union member and not on a public works project was not entitled to have health insurance benefits included in the calculation of his average weekly wages. Barofsky v. Lundermac Co., 4 Mass. Workers’ Comp. Rep. 135, 143-144 (1990). This court affirmed that decision on December 10, 1991, and stated that there were “policy considerations which favored both sides of this question, but they are more properly addressed to the Legislature which to date has left the definition of ‘average weekly wages’ unchanged despite a comprehensive revision of G. L. c. 152.” Borofsky’s Case, 411 Mass. 379, 381 (1991). It is presumed that the Legislature was aware of all of the these reviewing board decisions, and this court’s decision, at the time it enacted St. 1991, c. 398, on December 23, 1991. See Taylor’s Case, supra at 500, and cases cited.
The second paragraph comports with the Borofsky decision, in that it explicitly states that health insurance benefits are excluded from the determination of average weekly wages.7 However, the stated exception also affirms the previous decisions of the reviewing board, in that it references G. L. c. 149, §§ 26 and 27, the very sections on which the reviewing board relied for authority to include fringe benefits in the Machado and Lyons decisions.
Moreover, in a decision involving an injury sustained after the enactment of St. 1991, c. 398, Kelly v. Modem Continental, 17 Mass. Workers’ Comp. Rep. 172, 175 (2003), the reviewing board affirmed its earlier decisions and held that G. L. c. 149, §§ 26 and 27, applied to a union laborer injured on the job.8 Contrast Jerrett v. A. Ricciardelli & Sons, 8 Mass. Workers’ Comp. Rep. 389, 390 (1994) (denying union employee’s request to include benefits in calculation of average weekly wages where employee not employed on public project). We grant *367substantial deference to the reviewing board’s interpretation of G. L. c. 152. Gateley’s Case, 415 Mass. 397, 399 (1993) (“interpretation of a statute by the agency charged with primary responsibility for administering it is entitled to substantial deference”). Given the plain language of the statute, the timing of its revision, and presuming that the Legislature was aware of the line of decisions issued by the reviewing board before 1991, we cannot say that the reviewing board erred in applying the exception contained in G. L. c. 152, § 1 (1), to this case. Contrast Kszepka’s Case, 408 Mass. 843, 847 (1990) (reviewing board’s incorrect interpretation of statute not entitled to deference). In any event, were there any ambiguity in the statute, the reviewing board’s interpretation of the statute is entitled to our deference. Richards’s Case, 62 Mass. App. Ct. 701, 706 (2004), and cases cited.
The employer and the insurer argue that reading G. L. c. 149, §§ 26 and 27, to include fringe benefits for union employees constitutes discriminatory treatment between two classes of employees, favoring the union employee in violation of the equal protection clause of the Fourteenth Amendment to the United States Constitution and art. 1 of the Massachusetts Declaration of Rights. We need not address this issue because we conclude that the employer and the insurer do not have standing to raise it. “[0]ne may not claim standing ... to vindicate the constitutional rights of some third party,” Blixt v. Blixt, 437 Mass. 649, 661 (2002), cert. denied, 537 U.S. 1189 (2003), quoting Slama v. Attorney Gen., 384 Mass. 620, 624 (1981), because “[o]nly one whose rights are impaired by a statute can raise the question of its constitutionality, and he can object to the statute only as applied to him.” Blixt v. Blixt, supra, quoting Massachusetts Comm’n Against Discrimination v. Colangelo, 344 Mass. 387, 390 (1962).9
Conclusion. For the reasons set forth above, we conclude that *368the inclusion of fringe benefits in the detennination of average weekly wages for a unionized employee working on an all-union public works project is required under the plain language of G. L. c. 152, § 1 (1). The decision of the reviewing board is affirmed.
So ordered.
General Laws c. 152, §§ 34 and 35, set forth a minimum and maximum amount an injured employee may receive.
General Laws c. 149, § 27, also directs that “[w]hoever shall pay less” be punished or subject to a civil action or order and includes a private right of action by an aggrieved employee.
We have considered, but need not address, every argument the employer and the insurer have advanced in support of their interpretation of the statutes.
The legislative history the employer and insurer provided sheds no light on the specific statutory provision at issue in this case.
The 1991 legislation also addressed responsibilities of employers in furnishing notice of injuries and the commencement of workers’ compensation benefits. See, e.g., G. L. c. 152, §§ 6 & 7, as appearing in St. 1991, c. 398, §§ 18, 19, 20.
We need not address whether the list of excluded fringe benefits is exhaustive.
Kelly v. Modern Continental, 17 Mass. Workers’ Comp. Rep. 172, 175 (2003), also relied on McCarty v. Wilkinson & Co., 11 Mass. Workers’ Comp. Rep. 285 (1997), the reviewing board’s decision reversing the administrative judge’s decision to exclude fringe benefits from the determination of average weekly wages.
Moreover, even if they had standing, their cursory treatment of the issue is inadequate for appellate review. There is a presumption of the constitutionality of statutes and the parties have the “burden of proving the absence of any conceivable grounds upon which the statute may be supported,” which requires more than the assertion of general conclusions of fact and law. Merit Oil Co. v. Director of the Div. on the Necessaries of Life, 319 Mass. 301, 305 (1946). See Massachusetts Comm’n Against Discrimination v. Colangelo, 344 *368Mass. 387, 391 (1962).