
<PAGE>   1
                                                                   EXHIBIT 4.12


                          UNITED AIR SPECIALISTS, INC.

                1994 STOCK OPTION PLAN FOR NONEMPLOYEE DIRECTORS

        1.      PURPOSE. The purpose of this United Air Specialists, Inc. 1994
Stock Option Plan for Nonemployee Directors (the "Plan") is to enhance the
value of the shareholders' investment in United Air Specialists, Inc. (the
"Company") by encouraging those directors of the Company who are not employees
of the Company or any of its subsidiaries (the "Directors") to acquire or
increase and retain a financial interest in the Company, thereby also
encouraging the Directors to remain as directors.

        It is intended that stock options ("Nonqualified Stock Options" or
"Options"), other than incentive stock options as defined by Section 422 of
the Internal Revenue Code of 1986, as amended (the "Code"), may be granted
under the Plan.

        2.      ADMINISTRATION OF THE PLAN.

                (a)  GENERAL. This Plan shall be administered by the Board of
        Directors of the Company (the "Board") which, subject to and not
        inconsistent with the express provisions of the Plan, shall exercise all
        the power and authority specifically granted to it under the Plan or
        necessary and advisable, in the sole and absolute discretion of the
        Board, to the administration of the Plan.

                (b)  RULES AND INTERPRETATIONS. The Board shall have the
        authority to establish, adopt or revise such rules and regulations and
        to make all such determinations relating to the Plan as it may deem
        necessary or advisable for the administration of the Plan and in order
        to preserve the exemptions of the Plan and any Plan Options under Rule
        16b-3 promulgated by the Securities and Exchange Commission under the
        Securities Exchange Act of 1934, as amended (the "Exchange Act"), as
        such Rule may be amended from time to time or any successor rule
        thereto. The Board's interpretation of the Plan or any Option granted
        hereunder, and all decisions and determinations by the Board with
        respect to the Plan, shall be final, binding and conclusive on all
        parties. No member of the Board shall be personally liable for any
        action, failure to act, determination, interpretation or construction
        made in good faith with respect to the Plan or any Option or transaction
        hereunder.

                (c)  NO OTHER RIGHTS. Nothing contained in the Plan, or any
        Option granted pursuant to the Plan, shall confer upon any Director any
        right to continue as a director of the Company or limit in any way the
        right of the Company to terminate a Director's status as a director at
        any time.





                                      1
<PAGE>   2
        3.  THE STOCK. The shares of stock available for issuance pursuant to
the grant of Options under the Plan shall consist of 93,750 shares of the
Common Stock, no par value, (the "Common Shares"), of the Company, subject to
adjustment as provided in Section 11 hereof. Shares delivered by the Company
upon the exercise of Options may be, in whole or in part, either Common Shares
purchased by the Company in the open market and held in the treasury of the
Company or authorized and unissued Common Shares of the Company. Should an
Option (or a portion thereof) expire or terminate for any reason without being
exercised, the shares subject to the portion of such Option not so exercised
shall be available for subsequent grants under the Plan. Shares tendered or
withheld as payment pursuant to Sections 10 and 15 herein shall not be available
for issuance under the Plan.

        4.  EFFECTIVE DATE AND TERMINATION OF PLAN.  The Plan shall be
effective on September 1, 1994 (the "Effective Date"); provided, however, that,
if the Plan is not approved by the shareholders of the Company at the next
annual meeting of the Company's shareholders following the Effective Date, the
Plan and all Options granted hereunder shall be void and of no effect. No
Options may be exercised under the Plan prior to its approval by the
shareholders of the Company. The Plan shall terminate upon the earlier of (i)
September 1, 2004; or (ii) the date on which all shares available for issuance
under the Plan have been issued pursuant to the exercise of Options granted
hereunder; or (iii) the date as of which it is terminated by action of the
Board. No Options may be granted under the Plan after its termination date,
provided that the Options granted and outstanding on such date shall continue
to have force and effect in accordance with the provisions of the instruments
evidencing such Options.

        5.  GRANT, TERMS AND CONDITIONS OF OPTIONS.

            (a)  GRANT OF OPTIONS. Each current Director on the Effective Date
            shall be granted automatically, at the close of business on
            September 1, 1994, Nonqualified Stock Options to purchase 2,500
            Common Shares (at an option price of $2.00 per share). Similarly,
            each new Director shall be granted automatically, at the close of
            business on the date he or she first becomes a Director,
            Nonqualified Stock Options to purchase 2,500 Common Shares. After
            the Effective Date, each then serving Director of the Company shall
            be granted automatically, at the close of business on the date in
            any year upon which the Company's annual meeting of shareholders is
            held (or, if no annual meeting is held, as of the date designated
            for the annual meeting by the Company's Code of Regulations),
            Nonqualified Stock Options to purchase 1,000 Common Shares (other
            than a Director who was first elected a Director at that annual
            meeting.) Each Director receiving an Option may be referred to
            herein as an "Optionee". Each Option shall be embodied in an option
            agreement signed by the Optionee and the Company providing that the
            Option shall be subject to the provisions of this Plan and
            containing such other provisions as the Board may prescribe not
            inconsistent with the Plan.
 
            (b)  WHEN EXERCISABLE. Options shall be exercisable six months after
            the date of grant. No fractional shares shall be issued, and
            fractional shares remaining in any Option shall be rounded down to
            the nearest whole number of shares.



                                      2

<PAGE>   3
        (c)   PRICE. Except for Options granted on the Effective Date, the
exercise price per share of each Option shall be equal to the fair market value
of a Common Share on the date of grant, as determined under Section 8 hereof,
provided that the exercise price shall be subject to adjustment only as
provided in Section 11 hereof.

        (d)   TERMS OF OPTIONS. Options shall be effective on the date of grant
and shall be of a term of ten years from the date of grant unless earlier
terminated pursuant to the provisions of this Plan.

        (e)   HOLDING PERIOD. A period of at least six months must elapse from
the date of grant of any Option until the date of disposition of the Option
(other than upon exercise or conversion) or of any shares acquired upon
exercise of that Option.

6.      TERMINATION OF DIRECTOR STATUS.

        (a)   Except as otherwise provided in this Plan, an Optionee's Options
(i) are exercisable only by the Optionee, and (ii) if not exercisable by their
terms at the time the Optionee ceases to be a director of the Company, shall
immediately expire on the date the Optionee ceases to be a director of the
Company.

        (b)   Except as provided by this Subsection (6)(b), any Optionee's
Option which is exercisable by its terms at the time the Optionee ceases to be
a director of the Company must be exercised on or before the earlier of (i) one
year after the date the Optionee ceases to be a director of the Company or (ii)
the fixed expiration date of such Option, after which applicable period the
Option shall expire. If an Optionee's status as a director is terminated on
account of any act of fraud or intentional misrepresentation, or embezzlement,
misappropriation or conversion of the assets or opportunities of the Company or
any of its subsidiaries, all Options granted to such Optionee shall
automatically terminate as of the date of termination of the directorship.

        (c)   In the event of the death of the Optionee while a director of
the Company, each of that Optionee's unexercised Options (whether or not then
exercisable by its terms) shall become immediately exercisable by the
Optionee's estate for a period ending on the earlier of the fixed expiration
date of such Option or one year after the date of death, after which period
such Option shall expire. For purposes hereof, the estate of an Optionee shall
include the legal representatives thereof or any person who has acquired the
right to exercise an Option by reason of the death of the Optionee.

        (d)   In the event the Optionee ceases to be a director by reason of
permanent disability (as defined below), each of that Optionee's unexercised
Options (whether or not then exercisable by its terms) shall become exercisable
for a period ending on the earlier of the fixed expiration date of such Option
or one year from the date the Optionee ceases to be a Director, after which
period such Option shall expire. For purposes hereof, "permanent disability"
shall be deemed to be the inability of the Optionee to


                                       3
<PAGE>   4
        perform the duties of a director of the Company because of a physical
        or mental disability as evidenced by the opinion of a Company-approved
        doctor of medicine licensed to practice medicine in the United States
        of America.
        
        7.      TRANSFERABILITY OF OPTIONS. Any Option granted hereunder shall
be transferable only by will or the laws of descent and distribution and shall
be exercisable during the lifetime of the Optionee only by the Optionee or by
the Optionee's guardian or legal representative.

        8.      FAIR MARKET VALUE. The "fair market value" of a Common Share on
any relevant date for purposes of this Plan shall be the last reported sales
price of a Common Share on any stock exchange or on any trading system sponsored
by the National Association of Securities Dealers, Inc. on which the Shares are
trading on that date or, if there are no reported sales on such date, then the
last reported sales price on the next preceding day on which such a sale was
transacted. If the Common Shares are not then traded as described in the
preceding sentence, then the average of the closing bid and asked prices on the
specified date or on the last preceding day on which bid and asked prices were
reported, as quoted by such source as the Board may select, shall be used in
determining fair market value. If bid and asked prices are not then reported at
least weekly as described in the preceding sentence, then fair market value
shall be what the Board in good faith determines it to be.

        9.      COMPLIANCE WITH SECURITIES LAWS. Options granted and shares
issued by the Company upon exercise of Options shall be granted and issued only
in full compliance with all applicable federal and state securities laws, and
the Board may impose such conditions on transfer, restrictions and limitations
as it deems necessary and appropriate to assure compliance with such securities
laws.

       10.      METHOD OF EXERCISE. An Option granted under this Plan may be
exercised by written notice to the Board, signed by the Optionee or by such
other person as is entitled to exercise the Option. The notice of exercise shall
state the number of shares in respect of which the Option is being exercised,
and shall either be accompanied by the payment, in cash or in Common Shares as
provided hereafter, of the full option price for such shares or shall fix a date
(not more than ten business days from the date of such notice) for the payment
of the full option price for the shares being purchased. All or any portion of
the payment may be made by the transfer of Common Shares of the Company from the
Optionee to the Company or, in accordance with any applicable administrative
rules established by the Board, by directing that a portion of the Common Shares
to be issued upon exercise of the Option be withheld by the Company. Such shares
shall be valued for this purpose at their fair market value on the date they are
transferred to the Company or withheld as payment, determined in the same manner
as is provided in Section 8 hereof. A certificate or certificates for the Common
Shares of the Company purchased through the exercise of an Option shall be
issued in the regular course after the exercise of the Option and payment
therefor. During the option period, no person entitled to exercise any Option
granted under this Plan shall have any of the rights or privileges of a
shareholder with respect to any Common Shares issuable upon exercise of such
Option until certificates representing such shares shall have been issued and
delivered.



                                      4

<PAGE>   5
        11.  SHARE ADJUSTMENTS.  In the event there is any change in the
Company's Common Shares resulting from stock splits, stock dividends,
combinations or exchanges of shares, or other similar capital adjustments,
equitable proportionate adjustments shall automatically be made without further
action by the Board in (i) the number of Common Shares available for Option
grants under this Plan, (ii) the number of Common Shares subject to Options
granted under this Plan, and (iii) the option price of optioned shares.

        12.  MERGER, CONSOLIDATION OR SALE OF ASSETS.

             (a)  In the event the Company shall consolidate with, merge into, 
        or transfer all or substantially all of its assets to another
        corporation or corporations (a "successor corporation"), such successor
        corporation may obligate itself to continue this Plan and to assume all
        obligations under the Plan.  In the event that such successor
        corporation does not obligate itself to continue this Plan as above
        provided, the Plan shall terminate effective upon such consolidation,
        merger or transfer, and any Option previously granted hereunder shall
        terminate. If practical, the Company shall give each Optionee 20 days
        prior notice of any possible transaction which might terminate this
        Plan and the Options previously granted hereunder.
        
             (b)  The grant of Options under the Plan shall in no way affect the
        right of the Company to adjust, reclassify, reorganize or otherwise
        change its capital or business structure or to merge, consolidate,
        dissolve, liquidate or sell or transfer all or any part of its business
        or assets.

        13.  AMENDMENT OR TERMINATION. The Board may suspend or terminate the
Plan at any time. In addition, the Board may, from time to time, amend the Plan
in any manner, but may not, without shareholder approval, adopt any amendment
which would (a) increase the aggregate number of Common Shares issuable under
the Plan (except by operation of Section 11); (b) materially increase the
benefits to Optionees under the Plan; (c) materially modify the requirements as
to eligibility for participation in the Plan; or (d) in the opinion of counsel
to the Company require shareholder approval pursuant to any federal or state
law or rule or regulation promulgated thereunder. Notwithstanding the
foregoing, (i) the provisions of this Plan, including in particular Section 5,
governing the amount, price and timing of awards to Directors may not be
amended more than once every six months (with or without shareholder approval)
other than to comport with changes in the Code or the rules and regulations
thereunder, and (ii) no amendment may change, without the consent of the
Optionee (or such Optionee's or such Optionee's estates legal representative),
any Option previously granted to such Optionee under the Plan. If the Plan is
terminated, any unexercised Option shall continue to be exercisable in
accordance with its terms, except as provided in Paragraph 12 above.

        14.  COMPANY RESPONSIBILITY.  All expenses of this Plan, including the
cost of maintaining records, shall be borne by the Company. The Company shall
have no responsibility or liability (other than under applicable Securities
Acts) for any act or thing done or left undone with respect to the price, time,
quantity, or other conditions and circumstances of the purchase




                                      5
<PAGE>   6
of Common Shares under the terms of the Plan, so long as the Company acts in
good faith.

        15.     TAX WITHHOLDING. The Company shall be entitled to deduct from
any Common Shares delivered upon exercise of an Option the amount of all
applicable income, employment and other taxes required by law to be withheld
with respect to such exercise or may require the Optionee to pay such taxes to
the Company prior to and as a condition of the making of the exercise. So long
as such method is in accordance with any applicable administrative rules
established by the Board, an Optionee may pay the required withholding taxes
(i) by directing the withholding from any delivery of Common Shares by the
Company or (ii) by delivering to the Company Common Shares having a fair market
value, on the date of payment, equal to the amount of such taxes.

        16.     IMPLIED CONSENT. Every Optionee, by his acceptance of an Option
under this Plan, shall be deemed to have consented to be bound, on his or her
own behalf and on behalf of such Optionee's heirs, assigns and legal
representatives, by all of the terms and conditions of this Plan.

        17.     OHIO LAW TO GOVERN. This Plan shall be construed and
administered in accordance with and governed by the laws of the State of Ohio.



                                      6
