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                                                                EXHIBIT 4.10


                          UNITED AIR SPECIALISTS, INC.

                        1991 INCENTIVE STOCK OPTION PLAN

        1.  Effective Date of 1991 Plan.

            This plan known as the United Air Specialists, Inc. 1991 Incentive
Stock Option Plan (the "1991 Plan") shall become effective December 1, 1991, the
date fixed by the Board of Directors (the "Board") of United Air Specialists,
Inc. (the "Company") subject to approval, on or before November 30, 1992, by the
shareholders of the Company.

        2.  Purpose of 1991 Plan.

            This 1991 Plan is to afford an incentive to corporate officers and
key managerial personnel of the Company and of any subsidiaries of the Company
to acquire a proprietary interest in the Company and to enable the Company and
its subsidiaries to attract and retain such officers and key managerial
personnel.

        3.  The Stock.

            The stock which may be optioned and sold under the 1991 Plan is not
to exceed 225,000 shares of the Company's common stock, without par value, which
shares may be taken from the unissued but authorized shares of the Company's
common stock, from treasury shares, or from common stock purchased by the
Company from the open market. If for any reason an option under this 1991 Plan
expires in whole or in part, shares subject to such expired option may again be
subjected to an option under this 1991 Plan.

        4.  Eligibility.

            Options shall be granted only to key persons who (a) at the time of
the grant are corporate officers or key managerial personnel of the Company, or
of any of the Company's subsidiaries, and are eligible under the Internal
Revenue Code of 1986, as amended (the "Code"), to receive an "incentive stock
option" as defined in Section 422 of the Code, and (b) agree in writing to
remain in the employ of the Company, or of a Company subsidiary, for one year
from the date the option is granted. Under the terms of that statute, an option
may be granted at any time prior to December 1, 2001; and an option may be
exercised at any time within ten years after the granting of such option, except
that an option granted to any individual who owns more than ten percent of the
value of the total outstanding stock of the Company, or of any of the Company's
subsidiaries, must be exercised within five years after the granting of such
option. Whenever reference is made in this 1991 Plan to Section 422 or Section
424 of the Code, the 
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reference shall be to said Section as it is now in force or as it may be
hereafter be amended by an amendment which is applicable to this 1991 Plan.

                The terms "parent corporation" and "subsidiary corporation"
shall have the meanings given to them by Section 424 of the Code. With the
approval of the Board, the Committee designated pursuant to Section 10 of this
Plan (the "Committee") will determine or, if there is no Committee, the Board
will determine, the employees to whom options are to be granted and the number
of shares subject to each option. Participation in the 1991 Plan and the
exercise or non-exercise of any option granted are entirely voluntary on the
part of each employee to whom the option is offered.

                In determining stock ownership under this Section 4 and under
Section 5, the attribution rules of Section 424(d) of the Code shall apply.

                5.      Price.

                The price at which shares of the Company's common stock may be
purchased under the stock options granted hereunder shall be fixed by the
Committee with the approval of the Board, or if there is no Committee, by the
Board, and shall not be less than the following:

                (a)  110% of the fair market value of the stock at the time the
                option is granted (but not less than par value), with respect to
                an option granted to an individual who owns more than ten
                percent of the value of the total outstanding stock of the
                Company or of any parent corporation or subsidiary corporation
                of the Company; or

                (b)  the fair market value of the stock at the time the option
                is granted (but not less than par value), with respect to an
                option granted to individuals other than those described in
                subsection (a).

                The fair market value shall be what the Board, or the Committee
with the approval of the Board, in good faith determines it to be.

                6.      Time of Grant, Period of Options, and Right to
Exercise.

                Subject to the provisions and limitations of this 1991 Plan,
options may be granted by the Company at such time or times as may be determined
by the Committee with the approval of the Board or, if there is no Committee, by
the Board during the period this 1991 Plan is in effect.


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        The time or times when options granted hereunder or portions thereof
shall become exercisable, the expiration dates of the options, the manner of
their exercise, and the terms of payment of the option price shall be as
determined by the Committee with the approval of the Board, or if there is no
Committee, by the Board at or prior to the terms of the respective grants of
the options.

        Each option granted hereunder shall be exercisable during the lifetime
of the option grantee only by him and only if at all times during the period
beginning with the date of the granting of the option and ending on the day
three (3) months before the date he elects to exercise the option he was an
employee of either the Company, a parent or subsidiary corporation of the
Company, or a corporation or parent or subsidiary corporation of such
corporation issuing or assuming the option in a transaction to which Section
424(a) of the Code applies.

        In the event of the death of an option grantee at a time when he is 
entitled to exercise his option or any part thereof, his estate, or any person
who acquires the right to exercise his option by bequest or inheritance or by
reason of his death, shall have such rights to exercise his option as provided
in the grant of the option to him as shall be permissible in the case of
"incentive stock options" under the Code.

        7.  Non-Transferability.

        Options granted hereunder shall be personal to the employee, and shall
be non-assignable and non-transferable otherwise than by will and by the law of
descent and distribution.

        8.  Stock Dividends, Recapitalization, etc.

        In the event of a stock dividend paid in shares of the class of stock
subject to any option outstanding hereunder, or a recapitalization, a
reclassification, a split-up, or a combination of shares with respect to any
class of stock, the Committee with the approval of the Board or, if there is no
Committee, the Board shall have power to make appropriate adjustments of the
option price under such option and of the number of shares as to which such
option is then exercisable, to the end that the option grantee's proportionate
interest shall be maintained as before the occurrence of such event; and in any
such case, an appropriate adjustment shall also be made in the total number of
shares of stock reserved for the future granting of options under this 1991
Plan. Any such adjustment made by the Committee or the Board pursuant to this
1991 Plan shall be binding upon the holders of all unexpired option rights
outstanding hereunder. The Company shall


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give to each option grantee appropriate information as to any such adjustments.

        9.  Merger, Consolidation, Reorganization, Liquidation, etc.

        If the Company shall become a party to any corporation merger,
consolidation, major acquisition of property or stock, separation,
reorganization, or liquidation, the Committee with the approval of the Board,
or if there is no Committee, the Board shall have power to make arrangements,
which shall be binding upon the holders of unexpired option rights, for the
substitution of new options for any unexpired options then outstanding under
this 1991 Plan, or for the assumption of any such unexpired options, provided
that such arrangements shall meet the requirements of Section 424(a) of the
Code, or such similar provisions of the Code as may then be in effect.

        10.  Administration of 1991 Plan.

        This 1991 Plan shall be administered by the Stock Option Plan
Committee, which may be appointed from time to time by a resolution passed by a
majority of the whole Board, or, if no committee has been appointed by the Board
shall be administered by the Board. No option grantee hereunder shall be a
member of such Committee. All actions of the Committee shall be subject to the
approval of the Board.

        11.  Form of Option.

        Each option granted hereunder shall contain such provisions and
conditions in addition to those included in this 1991 Plan as the Committee
with the approval of the Board or, if there is no Committee, as the Board shall
deem advisable, provided that no such additional provisions or conditions shall
be inconsistent with this 1991 Plan or with the provisions of the Code dealing
with "incentive stock options."

        12.  Duration and Amendment of 1991 Plan.

        This 1991 Plan shall expire, unless sooner terminated by the Board of
Directors, on December 1, 2001. This 1991 Plan may be terminated at any time as
to all shares not then subject to outstanding options, by a resolution duly
adopted by the Board. Neither the expiration nor the termination of this 1991
Plan shall affect any option theretofore granted hereunder.

        The Board may from time to time amend the 1991 Plan, except that the
Board may not:


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            (1)  Increase the maximum number of shares for which options may be
            granted under the 1991 Plan, except as set forth in Section 8 above;

            (2)  Change the class of employees eligible to receive options;

            (3)  Change the limitation with respect to the minimum price at
            which shares may be optioned;

            (4)  Cause the Plan, as amended, to fail to qualify as a plan for
            the issuance of "incentive stock options" or to be inconsistent with
            the provisions of the Code dealing with "incentive stock options."

        No amendment of this 1991 Plan may, without the consent of the employee
to whom an option shall theretofore have been granted, adversely affect the
rights of such employee under such option.

        13.  Notices.

        All notices given, made, delivered, or transmitted to an employee by
the Company shall be deemed duly given when mailed first class mail, postage
prepaid, and addressed to the employee at the address last appearing on the
records of the Company. An employee may change the address as shown on the
records of the Company giving written notice thereof to the Company.

        14.  Approval of Appropriate Agencies.

        All options and shares issued hereunder shall be first registered with
or otherwise approved by all appropriate state and/or federal officers or
agencies, unless exempt from such registration provisions.




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