<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-11-27</periodOfReport>

    <issuer>
        <issuerCik>0000804212</issuerCik>
        <issuerName>AIRGAS INC</issuerName>
        <issuerTradingSymbol>ARG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001575929</rptOwnerCik>
            <rptOwnerName>Jones Douglas L.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O AIRGAS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>259 N. RADNOR-CHESTER ROAD, SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>RADNOR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19087</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Division President - West</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2013-11-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>13.1</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>109.80</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>13.1</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>429.3</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Acquired pursuant to Airgas, Inc.'s deferred compensation plan as a result of the deferral by the reporting person of a portion of his compensation.</footnote>
        <footnote id="F2">Each share of phantom stock represents the right to receive the cash value of one share of Airgas, Inc. common stock.</footnote>
        <footnote id="F3">Shares of phantom stock are payable in cash following the reporting person's termination of service with Airgas, Inc. or as determined by the reporting person in accordance with the terms and conditions of the plan.  The reporting person may transfer his phantom stock account into an alternative account under the plan at any time.</footnote>
        <footnote id="F4">Determined based on the dollar value of the reporting person's deferred compensation plan account and the closing price per share of Airgas, Inc. common stock on the date of the transaction.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Joan W. Schwartz, Attorney-in-Fact for Douglas L. Jones</signatureName>
        <signatureDate>2013-12-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
