Justice O’Connor
delivered the opinion of the Court.
We granted certiorari in this case to determine whether the Court of Appeals for the District of Columbia Circuit correctly concluded that the Food and Drug Administration’s longstanding interpretation of 21 U. S. C. § 346 was in conflict with the plain language of that provision. 474 U. S. 1018 (1985). We hold that, in light of the inherent ambiguity of the statutory provision and the reasonableness of the Food and Drug Administration’s interpretation thereof, the Court of Appeals erred. We therefore reverse.
I
A
The Food and Drug Administration (FDA) enforces the Federal Food, Drug, and Cosmetic Act (Act) as the designee of the Secretary of Health and Human Services. 21 U. S. C. § 371(a). See also 21 CFR §5.10 (1986). The Act seeks to ensure the purity of the Nation’s food supply, and accordingly bans “adulterated” food from interstate commerce. 21 U. S. C. § 331(a). Title 21 U. S. C. § 342(a) deems food to be “adulterated”
“(1) If it bears or contains any poisonous or deleterious substance which may render it injurious to health; but in case the substance is not an added substance such food shall not be considered adulterated under this clause if the quantity of such substance in such food does not ordinarily render it injurious to health; or (2)(A) if it bears or contains any added poisonous or added deleterious substance (other than [exceptions not relevant here]) which is unsafe within the meaning of section 346a(a) of this title . . . .”
As this provision makes clear, food containing a poisonous or deleterious substance in a quantity that ordinarily renders the food injurious to health is adulterated. If the harmful substance in the food is an added substance, then the food is deemed adulterated, even without direct proof that the food may be injurious to health, if the added substance is “unsafe” under 21 U. S. C. §346.
Section 346 states:
“Any poisonous or deleterious substance added to any food, except where such substance is required in the production thereof or cannot be avoided by good manufacturing practice shall be deemed to be unsafe for purposes of the application of clause (2)(A) of section 342(a) of this title; but when such substance is so required or cannot be so avoided, the Secretary shall promulgate regulations limiting the quantity therein or thereon to such extent as he finds necessary for the protection of public health, and any quantity exceeding the limits so fixed shall also be deemed to be unsafe for purposes of the application of clause (2)(A) of section 342(a) of this title. While such a regulation is in effect . . . food shall not, by reason of bearing or containing any added amount of such substance, be considered to be adulterated . . . .”
Any quantity of added poisonous or added deleterious substances is therefore “unsafe,” unless the substance is required in food production or cannot be avoided by good manufacturing practice. For these latter substances, “the Secretary shall promulgate regulations limiting the quantity therein or thereon to such extent as he finds necessary for the protection of public health.” It is this provision that is the heart of the dispute in this case.
The parties do not dispute that, since the enactment of the Act in 1938, the FDA has interpreted this provision to give it the discretion to decide whether to promulgate a § 346 regulation, which is known in the administrative vernacular as a “tolerance level.” Tolerance levels are set through a fairly elaborate process, similar to formal rulemaking, with evidentiary hearings. See 21 U. S. C. § 371(e). On some occasions, the FDA has instead set “action levels” through a less formal process. In setting an action level, the FDA essentially assures food producers that it ordinarily will not enforce the general adulteration provisions of the Act against them if the quantity of the harmful added substance in their food is less than the quantity specified by the action level.
B
The substance at issue in this case is aflatoxin, which is produced by a fungal mold that grows in some foods. Aflatoxin, a potent carcinogen, is indisputedly “poisonous” or “deleterious” under §§342 and 346. The parties also agree that, although aflatoxin is naturally and unavoidably present in some foods, it is to be treated as “added” to food under § 346. As a “poisonous or deleterious substance added to any food,” then, aflatoxin is a substance falling under the aegis of § 346, and therefore is at least potentially the subject of a tolerance level.
The FDA has not, however, set a § 346 tolerance level for aflatoxin. It has instead established an action level for aflatoxin of 20 parts per billion (ppb). In 1980, however, the FDA stated in a notice published in the Federal Register:
“The agency has determined that it will not recommend regulatory action for violation of the Federal Food, Drug, and Cosmetic Act with respect to the interstate shipment of corn from the 1980 crop harvested in North Carolina, South Carolina, and Virginia and which contains no more than 100 ppb aflatoxin . . . .” 46 Fed. Reg. 7448 (1981).
The notice further specified that such corn was to be used only as feed for mature, nonlactating livestock and mature poultry. Id., at 7447.
In connection with this notice, two public-interest groups and a consumer (respondents here) brought suit against the Commissioner of the FDA (petitioner here) in the United States District Court for the District of Columbia. Respondents alleged that the Act requires the FDA to set a tolerance level for aflatoxin before allowing the shipment in interstate commerce of food containing aflatoxin; that the FDA had employed insufficiently elaborate procedures to set its aflatoxin action level even if a tolerance level was not required; and that the FDA’s decision to grant the 1980 exemption from the action level independently violated the Act and the FDA’s own regulations.
On a motion for summary judgment, the District Court deferred to the FDA’s interpretation of §346, and therefore ruled that the FDA need not establish a tolerance level for aflatoxin before allowing the shipment of aflatoxin-tainted corn in interstate commerce. The District Court also ruled against respondents on their other claims.
The Court of Appeals reversed the District Court’s conclusion as to the proper interpretation of § 346. 244 U. S. App. D. C. 279, 757 F. 2d 354 (1985). The Court of Appeals determined that Congress had spoken directly and unambiguously to the precise question at issue:
“The presence of the critical word ‘shall’ plainly suggests a directive to the Secretary to establish a tolerance, if a food with an unavoidable . . . deleterious substance is to be considered unadulterated.
“It is . . . clear from the structure of the sentence at issue here that the phrase relied upon by the Secretary simply does not modify the pivotal word ‘shall.’” Id., at 282, 283, 757 F. 2d, at 357, 358.
After examining the entirety of § 346, the Court of Appeals also concluded that, since tolerance levels make food with added harmful substances unadulterated, tolerance levels were necessary before food could be judged unadulterated. Id., at 283, 757 F. 2d, at 358.
The Court of Appeals considered none of the other issues before the District Court, and therefore only the § 346 issue is before this Court.
II
The FDA’s longstanding interpretation of the statute that it administers is that the phrase “to such extent as he finds necessary for the protection of public health” in § 346 modifies the word “shall.” The FDA therefore interprets the statute to state that the FDA shall promulgate regulations to the extent that it believes the regulations necessary to protect the public health. Whether regulations are necessary to protect the public health is, under this interpretation, a determination to be made by the FDA.
Respondents, in contrast, argue that the phrase “to such extent” modifies the phrase “the quantity therein or thereon” in §346, not the word “shall.” Since respondents therefore view the word “shall” as unqualified, they interpret § 346 to require the promulgation of tolerance levels for added, but unavoidable, harmful substances. The FDA under this interpretation of § 346 has discretion in setting the particular tolerance level, but not in deciding whether to set a tolerance level at all.
Our analysis must begin with Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837 (1984). We there stated:
“First, always, is the question whether Congress has directly spoken to the precise question at issue. If the intent of Congress is clear, that is the end of the matter, for the court, as well as the agency, must give effect to the unambiguously expressed intent of Congress. If, however, the court determines Congress has not directly addressed the precise question at issue, the court does not simply impose its own construction on the statute, as would be necessary in the absence of an administrative interpretation. Rather, if the statute is silent or ambiguous with respect to the specific issue, the question for the court is whether the agency’s answer is based on a permissible construction of the statute. . . . [A] court may not substitute its own construction of a statutory provision for a reasonable interpretation made by the administrator of an agency.” Id., at 842-844.
While we agree with the Court of Appeals that Congress in § 346 was speaking directly to the precise question at issue in this case, we cannot agree with the Court of Appeals that Congress unambiguously expressed its intent through its choice of statutory language. The Court of Appeals’ reading of the statute may seem to some to be the more natural interpretation, but the phrasing of § 346 admits of either respondents’ or petitioner’s reading of the statute. As enemies of the dangling participle well know, the English language does not always force a writer to specify which of two possible objects is the one to which a modifying phrase relates. A Congress more precise or more prescient than the one that enacted § 346 might, if it wished petitioner’s position to prevail, have placed “to such extent as he finds necessary for the protection of public health” as an appositive phrase immediately after “shall” rather than as a free-floating phrase after “the quantity therein or thereon.” A Congress equally fastidious and foresighted, but intending respondents’ position to prevail, might have substituted the phrase “to the quantity” for the phrase “to such extent as.” But the Congress that actually enacted § 346 took neither tack. In the absence of such improvements, the wording of § 346 must remain ambiguous.
The FDA has therefore advanced an interpretation of an ambiguous statutory provision.
“This view of the agency charged with administering the statute is entitled to considerable deference; and to sustain it, we need not find that it is the only permissible construction that [the agency] might have adopted but only that [the agency’s] understanding of this very ‘complex statute’ is a sufficiently rational one to preclude a court from substituting its judgment for that of [the agency]. Train, Inc. v. NRDC, 421 U. S. 60, 75, 87 (1975) . . . .” Chemical Manufacturers Assn. v. Natural Resources Defense Council, Inc., 470 U. S. 116, 125 (1985).
We find the FDA’s interpretation of § 346 to be sufficiently rational to preclude a court from substituting its judgment for that of the FDA.
To read §346 as does the FDA is hardly to endorse an absurd result. Like any other administrative agency, the FDA has been delegated broad discretion by Congress in any number of areas. To interpret Congress’ statutory language to give the FDA discretion to decide whether toleranee levels are necessary to protect the public health is therefore sensible.
Nor does any other portion of § 346 prohibit the FDA from allowing the shipment of aflatoxin-tainted food without a tolerance level, despite the Court of Appeals’ conclusion to the contrary. The Court of Appeals stated:
“Since the existence of a regulation operates to render the food legally unadulterated, the statute, in our view, plainly requires the establishment by regulation of tolerances before aflatoxin-tainted corn may lawfully be shipped in interstate commerce.” 244 U. S. App. D. C., at 283, 757 F. 2d, at 358.
The premise of the Court of Appeals is of course correct: the Act does provide that when a tolerance level has been set and a food contains an added harmful substance in a quantity below the tolerance level, the food is legally not adulterated. But one cannot logically draw from this premise, or from the Act, the Court of Appeals’ conclusion that food containing substances not subject to a tolerance level must be deemed adulterated. The presence of a certain premise (i. e., tolerance levels) may imply the absence of a particular conclusion (i. e., adulteration) without the absence of the premise implying the presence of the conclusion. For example, the presence of independent and adequate state-law grounds in the decision of a state supreme court means this Court has no jurisdiction over the case, but the absence of independent and adequate state grounds does not mean that this Court necessarily has jurisdiction. The Act is silent on what specifically to do about food containing an unavoidable, harmful, added substance for which there is no tolerance level; we must therefore assume that Congress intended the general provisions of § 342(a) to apply in such a case. Section 342(a) thus remains available to the FDA to prevent the shipment of any food “[i]f it bears or contains any poisonous or deleterious substance which may render it injurious to health.” See generally United States v. Lexington Mill & Elevator Co., 232 U. S. 399, 411 (1914) (discussing proper interpretation of the language that became § 342(a)).
The legislative history of the Act provides no single view about whether Congress intended § 346 to be mandatory or permissive with respect to tolerance levels. Compare, e. g., Confidential House Committee Print 2, on Interstate and Foreign Commerce, 75th Cong., 1st Sess., S. 5, §406(a), reprinted in 5 Legislative History of the Federal Food, Drug, and Cosmetic Act and its Amendments 767, 792 (Dept. of Health, Education, and Welfare 1979) (changing, without explanation, words “is authorized to” to “shall” in relevant provision), with H. R. Rep. No. 2139, 75th Cong., 3d Sess. 6 (1938) (stating that, under the Act, “the establishment of tolerances is authorized”) (emphasis added). A clearer indication of Congress’ intentions with regard to tolerance levels occurred in 1954, when Congress condemned the cumbersomeness of the tolerance-level procedure as applied to pesticides. Congress fashioned a more streamlined procedure for those and other deliberately added substances. See 21 U. S. C. §346a. But in revisiting §346, Congress did not change the procedures governing unintentionally added substances like aflatoxin. This failure to change the scheme under which the FDA operated is significant, for a “congressional failure to revise or repeal the agency’s interpretation is persuasive evidence that the interpretation is the one intended by Congress.” NLRB v. Bell Aerospace, Co., 416 U. S. 267, 275 (1974). See FDIC v. Philadelphia Gear Corp., ante, at 437; Zenith Radio Corp. v. United States, 437 U. S. 443, 457 (1978). In sum, although the legislative history is not unambiguous, it certainly is no support for assertions that the FDA’s interpretation of §346 is insufficiently rational to warrant our deference.
Finally, we note that our interpretation of § 346 does not render that provision superfluous, even in light of Congress’ decision to authorize the FDA to “promulgate regulations for the efficient enforcement of [the] Act.” 21 U. S. C. § 371(a). Section 346 gives the FDA the authority to choose whatever tolerance level is deemed “necessary for the protection of public health,” and food containing a quantity of a required or unavoidable substance less than the tolerance level “shall not, by reason of bearing or containing any added amount of such substance, be considered to be adulterated.” Section 346 thereby creates a specific exception to § 342(a)’s general definition of adulterated food as that containing a quantity of a substance that renders the food “ordinarily . . . injurious to health.” Simply because the FDA is given the choice between employing the standard of § 346 and the standard of § 342(a) does not render § 346 superfluous.
For the reasons set forth, the judgment is reversed, and the case is remanded to the Court of Appeals for the District of Columbia Circuit for further proceedings consistent with this opinion.
Reversed.
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