- What the Official Domain Chart Actually Says
- Domain 1: Scientific Knowledge of Biological Agents
- Domain 2: Potential Health Effects and Risks
- Domain 3: Principles of Microbial Investigation
- Domain 4: Principles of Microbial Sampling
- Domain 5: Principles of Microbial Remediation
- Domain 6: Health & Safety
- Reference Books and Edition Options
- Exam Format and How Domains Appear
- Sequencing the Six Domains in Your Prep
- Frequently Asked Questions
- The CMI exam covers six official domains, from biological agents through Health & Safety, per ACAC's chart approved February 21, 2024.
- ACAC publishes no domain weights, so any percentage you see for CMI domains is a publisher estimate, not an official figure.
- The exam is 100 multiple-choice questions, three hours, closed book, with a 75% passing score.
- Health & Safety is a separate domain on the formal chart, even though the shorter handbook summary omits it.
What the Official Domain Chart Actually Says
The Council-certified Microbial Investigator (CMI) credential is issued by the American Council for Accredited Certification (ACAC) through its CMI Certification Board, and the program is accredited by the Council for Engineering and Scientific Specialty Boards (CESB). The authoritative source for what the exam covers is the ACAC CMC and CMI Examinations domain chart, approved February 21, 2024. That chart lists six exam domains and, beneath them, columns of subdomains numbered 1 through 7.
That numbering trips up many candidates. The columns numbered 1 through 7 are subdomains, not seven additional exam domains. The exam has six domains, full stop. No separately labeled "2026 version" of the exam has been verified, so the February 2024 chart is the current formal statement of exam objectives.
Equally important: ACAC does not publish percentage weights for these domains. You will see prep sites assign allocations such as 25/25/20/15/15, but those describe how a publisher built its practice material, not how the real exam is blended. Because no official weighting exists, you cannot safely skip a domain on the theory that it is "low value." For a broader orientation before diving in, see our overview of what CMI certification is, and keep the CMI study guide handy as your master plan.
| # | Official Domain | Core Question It Answers |
|---|---|---|
| 1 | Scientific knowledge of biological agents | What are these organisms and how do they behave? |
| 2 | Potential health effects and risks associated with exposure | What can exposure do to people? |
| 3 | Principles of Microbial Investigation | How do I find and interpret the problem? |
| 4 | Principles of Microbial Sampling | How do I collect defensible data? |
| 5 | Principles of Microbial Remediation | How is contamination properly corrected? |
| 6 | Health & Safety | How do I protect people and myself? |
Domain 1: Scientific Knowledge of Biological Agents
This domain is the foundation for everything else. It asks whether you understand the biology behind the materials you will be sampling and interpreting. Expect questions that assume you can reason about how microorganisms grow, spread, and persist in built environments rather than recite isolated definitions.
Domain 1 Focus Areas
Candidates should be comfortable with the biological principles that drive indoor microbial problems.
- Fungi, bacteria, and other biological contaminants relevant to indoor environments
- Conditions that support growth, especially moisture and building materials
- Reproductive structures and how particles become airborne
- Terminology you must read and use precisely in reports
The relevant references here include the AIHA field guide on determining biological contaminants and ACGIH's Bioaerosols: Assessment and Control. Candidates with a microbiology background often find this domain familiar, while those coming from construction or restoration should budget extra time for vocabulary and organism behavior. Practice questions on our main practice test site are a good way to find out which side of that line you fall on.
Domain 2: Potential Health Effects and Risks Associated with Exposure
Domain 2 connects biology to people. It covers how exposure to biological agents may affect occupants and what factors influence risk. A strong answer on this domain demonstrates restraint: responsible investigators describe potential health effects and risk factors without overclaiming causation in a specific individual.
Domain 2 Focus Areas
Expect scenario-flavored questions that test judgment about exposure and vulnerability.
- Routes of exposure and the general categories of health effects associated with biological agents
- Why some occupants are more susceptible than others
- The limits of what an investigator can and cannot conclude about health outcomes
- How risk information shapes communication with clients and occupants
This domain also underpins the professional boundary of the credential. A microbial investigator characterizes conditions and exposures; diagnosing illness belongs to medical professionals. Questions that tempt you to attribute symptoms to a specific contaminant are usually probing exactly that boundary.
Domain 3: Principles of Microbial Investigation
Investigation is where the credential's identity as an investigator shows. This domain deals with the logic of assessing a property: understanding the building, the water or moisture history, the complaint, and how to plan an evaluation before any sample is taken.
Domain 3 Focus Areas
Think of this as the front end of an engagement, before and around sampling.
- Building science fundamentals that explain moisture intrusion and migration
- Visual and physical assessment and documentation practices
- Forming and testing hypotheses about sources and pathways
- Interpreting findings in context rather than in isolation
The eligibility rules reinforce why this domain matters: candidates must demonstrate designing, conducting, and interpreting microbial sampling regimens, and must submit four project sheets as part of the application. Those project sheets are essentially real-world Domain 3 and Domain 4 evidence. If you are still sorting out whether you qualify, read CMI requirements and eligibility before investing heavily in study.
Domain 4: Principles of Microbial Sampling
Sampling is the technical heart of the exam and the skill the application process screens for most directly. The pathway into the credential requires either documented microbial sampling experience plus a related two-year degree (or 15 qualifying college credits), or a high school diploma/GED plus two years of consulting experience in microbial sampling. ACAC's eligibility FAQ adds that at least one full year of actual field experience is required regardless of college credits.
Domain 4 Focus Areas
Questions here reward process discipline and an understanding of what each sampling approach can and cannot tell you.
- Selecting sampling strategies that fit the question being asked
- Air, surface, and bulk approaches and their respective strengths and limitations
- Controls, chain of custody, and documentation that make results defensible
- Interpreting laboratory results and comparing indoor with outdoor conditions
Because most candidates qualify through field sampling experience, this domain often feels most comfortable. The risk is overconfidence: field habit is not the same as being able to justify a method against a standard. Calibrate with practice questions and compare your reasoning against the reference texts rather than against what your employer has always done. For a sense of how demanding the whole exam is, see how hard the CMI exam is.
Domain 5: Principles of Microbial Remediation
Even though the credential is an investigator credential, the exam expects you to understand how problems are corrected, because your protocols, reports, and clearance judgments interact with remediation work. The two IICRC documents on the reference list anchor this domain.
Domain 5 Focus Areas
Know the logic of remediation well enough to evaluate and specify it.
- Water damage restoration principles from ANSI/IICRC S500
- Mold remediation principles from ANSI/IICRC S520 and the IICRC R520 reference guide
- Source correction, containment, and the sequence of remediation steps
- How post-remediation evaluation confirms that conditions are acceptable
One edition detail deserves attention. ACAC's handbook lists the third edition of S520 and R520 (2015) or the fourth edition, ANSI/IICRC S520-2024. Both appear as options, so confirm in the current handbook which edition you are studying and be alert to terminology differences between editions.
Domain 6: Health & Safety
Health & Safety is the domain most likely to be underestimated, partly because the handbook's shorter five-topic summary omits it. The formal chart includes it as a full domain, so prepare for it as one. It governs how investigators protect themselves, occupants, and the public during assessment work.
Domain 6 Focus Areas
The official reference list points to respiratory protection as a core thread.
- The OSHA Small Entity Compliance Guide for the Respiratory Protection Standard (2011)
- NIOSH Respirator Selection Logic (2004) and how selection decisions are structured
- Hazard recognition during inspections and sampling
- Protecting occupants from disturbance and cross-contamination during assessment
Note a subtle study point: although the CMI bibliography lists the OSHA 2011 guide, the CMI detailed study-guide section does not provide a section or page subset for it. That means you cannot rely on a short excerpt; plan to read the guide's logic broadly. Candidates who treat this domain as common sense tend to lose points on selection-logic and program-requirement questions.
Reference Books and Edition Options
ACAC's handbook lists the following references. These are bibliographic listings with stated edition alternatives; they do not by themselves establish an exam revision.
| Reference | Edition Information | Most Relevant Domains |
|---|---|---|
| ACGIH, Bioaerosols: Assessment and Control | First edition (1999) or second edition (2024) | 1, 2, 4 |
| AIHA, Field Guide for the Determination of Biological Contaminants | Second edition (2005) | 1, 3, 4 |
| ANSI/IICRC S500, Water Damage Restoration | 2021, fifth edition | 3, 5 |
| ANSI/IICRC S520 and IICRC R520, Mold Remediation | 2015 third editions, or ANSI/IICRC S520-2024 fourth edition | 3, 5 |
| OSHA, Respiratory Protection Standard compliance guide | 2011 | 6 |
| NIOSH, Respirator Selection Logic | 2004 | 6 |
Key Takeaway
The handbook's selected study-guide pages are illustrative, not exhaustive. ACAC explicitly warns that candidates need comprehensive knowledge of the reference texts, and the formal chart, not the excerpts, defines exam objectives.
Exam Format and How Domains Appear
The CMI exam is administered through Kryterion, Inc. as a computer-delivered written examination with 100 multiple-choice questions, each with at least four answer options. You get three hours, the exam is closed book, and the passing score is 75%, set using the Modified Angoff method. The split between scored and unscored questions is not published, and no separate practical assessment is described.
- Delivery: proctored at a testing center or online with video proctoring.
- Online requirements: your own computer, an approved webcam, a private location, and at least 2 Mbps upload and download; phones and tablets are excluded.
- Testing center ID: two government-issued IDs, including one with a photo.
- Question handling: a fixed question set with randomized question and answer order; adaptive testing is not described, and calculator permission is unverified.
- Results: immediate electronic score and an emailed result.
ACAC's exams page says to arrive 15 minutes early, while the handbook says 30. Because the issuer's own materials disagree, plan for the longer window. For scheduling mechanics, see CMI exam dates and scheduling, and for the scoring threshold in detail, read the CMI passing score explained.
Cost-wise, the examination fee is $100 and the certification fee is $400, for $500 over the initial two-year term. The $400 includes a $100 nonrefundable processing fee, which is not an additional charge. A retake is $100 and renewal is $400. Details are in the CMI certification cost breakdown.
Sequencing the Six Domains in Your Prep
Because ACAC publishes no weights, the sensible approach is to sequence by dependency and by your own gaps rather than by guessed percentages. Biology and health effects come first because later questions assume them; investigation and sampling follow; remediation and Health & Safety close the loop. Adjust the order if your background already covers a block.
Domains 1 and 2
- Build vocabulary and organism behavior from the AIHA field guide and ACGIH text
- Practice stating health-effect risks without overclaiming causation
Domains 3 and 4
- Rehearse investigation logic and sampling method selection against your own project sheets
- Write out what each method can and cannot answer
Domains 5 and 6
- Work through the S500 and S520 principles and your chosen edition differences
- Study respirator selection logic and the OSHA compliance guide, not just excerpts
Mixed Review
- Take timed sets of 100 questions to rehearse the three-hour format
- Revisit whichever domain your misses cluster in
A single review pass is not enough for a closed-book exam that draws on multiple full-length references. Pair this schedule with the CMI cheat sheet for last-day review, and use the practice tests on the main site to tag every miss to a domain so your final week targets real weaknesses.
Frequently Asked Questions
Six: scientific knowledge of biological agents, potential health effects and risks associated with exposure, principles of microbial investigation, principles of microbial sampling, principles of microbial remediation, and Health & Safety. The numbered columns 1 through 7 on ACAC's chart are subdomains, not additional domains.
That cannot be established. ACAC publishes no official percentage weights for the six domains. Any allocation you see on a prep site reflects that publisher's practice material, so prepare across all six rather than betting on one.
The handbook's shorter summary omits the separate Health & Safety domain that appears on the formal chart approved February 21, 2024. The formal chart controls, so study Health & Safety as its own domain, including respirator selection logic.
It is closed book, with 100 multiple-choice questions over three hours, and no separate practical assessment is described. Your hands-on experience is evaluated through the application, including four project sheets and experience verification.
A 75% passing score, established using the Modified Angoff method. Candidates receive an immediate electronic score and an emailed result. See the passing score guide for how this differs from the published pass ratio.
Whether you are building toward the credential for the first time or deciding if it fits your career, the six domains above are the map. Knowing what each one tests, and which references stand behind it, is the most reliable way to turn scattered field experience into a passing result. If you are weighing the investment, our ROI analysis walks through the decision without relying on unverified earnings claims.