100+ Free CES Practice Questions

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Which of the following best describes the concept of 'sustainability' as defined in the Brundtland Report (1987)?

Using technology to maximize economic growth without environmental regulation

Protecting wilderness areas from any form of human development

Replacing all fossil fuels with renewable energy within 10 years

Development that meets the needs of the present without compromising the ability of future generations to meet their own needs

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2026 Statistics

Key Facts: CES Exam

100

Total Exam Questions

NREP CES program

75%

Passing Score

NREP

2 hours

Exam Duration

NREP

$300

Total Fee (Application + Exam)

NREP 2026

32%

Air Content Weight (largest section)

NREP CES Content Outline

SOC 19-2041

BLS Occupation (Environmental Scientists & Specialists)

US BLS OEWS

The NREP CES exam is a 100-question, 2-hour, closed-book online multiple-choice exam with a 75% passing score ($300 total fee: $100 application + $200 exam). It covers seven content areas: Air (32%), Water (24%), Waste (24%), Human Health & Environmental Ethics (7%), Green Chemistry & Radiation (6%), Pesticides (6%), and Brownfields (1%). Eligibility requires a bachelor's degree in an environmental field OR 3 years of relevant experience, plus 3 additional years of environmental science work experience. The credential maps to BLS SOC 19-2041 Environmental Scientists & Specialists.

Sample CES Practice Questions

Try these sample questions to test your CES exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1Which of the six NAAQS criteria pollutants is measured at both a 24-hour and an annual average standard by the EPA?

A.Carbon monoxide (CO)

B.Ozone (O3)

C.Fine particulate matter (PM2.5)

D.Lead (Pb)

Explanation: PM2.5 has both a 24-hour standard (35 µg/m³) and an annual standard (12 µg/m³ for primary). CO has 1-hour and 8-hour standards. Ozone has only an 8-hour standard. Lead uses a rolling 3-month average. PM2.5's dual averaging periods reflect both acute and chronic health effects of fine particles.

2The six NAAQS criteria pollutants regulated under the Clean Air Act include all of the following EXCEPT:

A.Sulfur dioxide (SO2)

B.Nitrogen dioxide (NO2)

C.Benzene

D.Lead (Pb)

Explanation: The six NAAQS criteria pollutants are CO, NO2, SO2, O3, PM (PM2.5 and PM10), and Pb. Benzene is a hazardous air pollutant (HAP) regulated under CAA Section 112, not a criteria pollutant with an ambient NAAQS standard. Criteria pollutants are selected because they are widespread, come from numerous sources, and have well-documented health effects.

3A stable atmospheric condition in which a warm air layer overlies cooler air near the surface is called a:

A.Adiabatic lapse

B.Temperature inversion

C.Atmospheric mixing

D.Convective overturning

Explanation: A temperature inversion occurs when temperature increases with altitude rather than decreasing — the opposite of the normal environmental lapse rate. Inversions trap pollutants near the surface because the warm upper layer acts as a lid, preventing vertical mixing. Inversions are responsible for many urban smog episodes and elevated ground-level pollutant concentrations.

4Ground-level ozone is primarily formed by the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of:

A.Ultraviolet sunlight

B.High relative humidity

C.Particulate matter

D.Carbon monoxide

Explanation: Ground-level (tropospheric) ozone is a secondary pollutant formed through photochemical reactions. NOx and VOCs react in the presence of ultraviolet (UV) sunlight to produce ozone. This is why ozone concentrations peak on hot, sunny, calm days — conditions that maximize UV flux and minimize dispersion. Ozone is not directly emitted but is formed in the atmosphere.

5The Gaussian dispersion model for stack emissions assumes that pollutant concentrations downwind follow which distribution in both the horizontal and vertical directions?

A.Uniform distribution

B.Log-normal distribution

C.Normal (Gaussian) distribution

D.Exponential decay distribution

Explanation: Gaussian dispersion models (e.g., AERMOD) assume that pollutant concentrations spread in a bell-shaped normal (Gaussian) distribution both horizontally (y) and vertically (z) from the plume centerline. The dispersion coefficients (σy and σz) characterize plume spread and depend on atmospheric stability class and downwind distance.

6Stratospheric ozone depletion is primarily caused by the catalytic destruction of ozone by which compound?

A.Carbon dioxide (CO2)

B.Chlorofluorocarbons (CFCs)

C.Sulfur hexafluoride (SF6)

D.Methane (CH4)

Explanation: Chlorofluorocarbons (CFCs) migrate to the stratosphere where UV radiation breaks them down to release chlorine radicals. Each chlorine radical can destroy thousands of ozone molecules through catalytic cycles (Cl + O3 → ClO + O2; ClO + O → Cl + O2). The Montreal Protocol (1987) phased out CFCs to protect the ozone layer.

7Acid rain is formed primarily when sulfur dioxide (SO2) and nitrogen oxides (NOx) react with water in the atmosphere to produce:

A.Carbonic acid and nitrous acid

B.Sulfuric acid and nitric acid

C.Hydrochloric acid and sulfurous acid

D.Phosphoric acid and nitric acid

Explanation: In the atmosphere, SO2 oxidizes to SO3, which dissolves in water droplets to form sulfuric acid (H2SO4). NOx oxidizes to NO2 and then to nitric acid (HNO3). These strong acids lower precipitation pH below 5.6 (normal CO2-acidified rain), damaging aquatic ecosystems, forests, and building materials. Power plants and transportation are the major NOx/SO2 sources.

8Pasquill-Gifford stability classes range from A through F (or G). Class A represents which atmospheric condition?

A.Extremely stable conditions (nighttime, low wind)

B.Extremely unstable conditions (strong insolation, low wind)

C.Neutral atmospheric stability

D.Moderately stable conditions (light winds at night)

Explanation: Pasquill-Gifford Class A represents the most unstable atmospheric condition — characterized by strong solar insolation (daytime heating) and light winds. Unstable conditions promote vigorous vertical mixing and rapid plume rise, resulting in faster dilution of pollutants. Class F (or G) represents the most stable conditions (nighttime, calm winds) that trap pollutants near the surface.

9Which greenhouse gas has the highest global warming potential (GWP) over a 100-year time horizon among those listed?

A.Carbon dioxide (CO2)

B.Methane (CH4)

C.Nitrous oxide (N2O)

D.Sulfur hexafluoride (SF6)

Explanation: SF6 has a GWP of approximately 23,500 over 100 years — far exceeding CO2 (GWP = 1), CH4 (GWP ≈ 28–36), and N2O (GWP ≈ 273). SF6 is used as an electrical insulator in high-voltage equipment. Although emitted in tiny quantities compared to CO2, each molecule is an extremely potent and long-lived greenhouse gas.

10The Clean Air Act New Source Review (NSR) program requires major new or modified stationary sources in nonattainment areas to obtain which type of permit?

A.Title V Operating Permit

B.Prevention of Significant Deterioration (PSD) Permit

C.Nonattainment New Source Review (NNSR) Permit

D.General Permit

Explanation: In nonattainment areas (areas that violate NAAQS), major new or modified stationary sources must obtain a Nonattainment NSR (NNSR) permit. NNSR requires the source to install Lowest Achievable Emission Rate (LAER) controls and obtain offsets. In attainment or unclassifiable areas, the parallel program is Prevention of Significant Deterioration (PSD), which requires Best Available Control Technology (BACT).

About the CES Exam

The NREP Certified Environmental Scientist (CES) establishes a professional's understanding of the basic principles of environmental science, including air quality, water resources, waste management, ecology, green chemistry, radiation, toxicology, and environmental regulations. It is an entry-level credential suited to recent graduates and early-career professionals in environmental science, engineering, health, and consulting.

Questions

100 scored questions

Time Limit

2 hours

Passing Score

75%

Exam Fee

$300 (NREP (National Registry of Environmental Professionals))

CES Exam Content Outline

32%

Air

NAAQS criteria pollutants, atmospheric stability (Pasquill-Gifford classes), Gaussian dispersion, temperature inversions, ozone (tropospheric and stratospheric), greenhouse gases, acid rain, climate change, mixing height, CAA overview

24%

Water

Hydrologic cycle, groundwater (aquifers, water table, Darcy's Law), surface water quality (BOD, DO, pH, turbidity), eutrophication (N & P), wastewater treatment (activated sludge), SDWA/MCLs/MCLGs, CWA Sections 402 and 404, UIC program

24%

Waste

RCRA Subtitle C hazardous waste characteristics (ICRT), TCLP, generator categories (VSQG/SQG/LQG), Subtitle D MSW landfills (composite liner requirement), waste hierarchy, universal waste, used oil (40 CFR 279), waste-to-energy incineration

Human Health and Environmental Ethics

Dose-response relationships, RfD, NOAEL, hazard quotient, cancer slope factor, bioaccumulation, biomagnification, Kow, EDCs, LD50/LC50, lead exposure pathways, RBCA, precautionary principle, environmental ethics

Green Chemistry and Radiation

12 Principles of Green Chemistry, atom economy, design for degradation, life cycle assessment (LCA), sustainability (Brundtland definition), ionizing vs non-ionizing radiation, alpha/beta/gamma penetration, rem/sievert/gray, radioactive half-life, ALARA principle

Pesticides

FIFRA registration, pesticide classes (organophosphates, carbamates, pyrethroids, neonicotinoids), mechanisms of action (AChE inhibition, sodium channel disruption), bioaccumulation of chlorinated pesticides, DDT, POPs (Stockholm Convention)

Brownfields

Brownfield definition (Small Business Liability Relief and Brownfields Revitalization Act), EPA Brownfields Program, vapor intrusion pathway, RBCA, site reuse and redevelopment

How to Pass the CES Exam

What You Need to Know

Passing score: 75%
Exam length: 100 questions
Time limit: 2 hours
Exam fee: $300

Keys to Passing

Complete 500+ practice questions
Score 80%+ consistently before scheduling
Focus on highest-weighted sections
Use our AI tutor for tough concepts

CES Study Tips from Top Performers

1Air quality questions dominate (32% of the exam) — prioritize NAAQS criteria pollutants, their averaging periods, atmospheric stability classes, and photochemical ozone formation from VOCs + NOx in sunlight

2Water and Waste together equal Air (24% each) — know the four RCRA hazardous waste characteristics (ICRT), generator categories (VSQG/SQG/LQG and their thresholds), and the Streeter-Phelps DO sag curve

3Remember the distinction between primary NAAQS (protect public health) and secondary NAAQS (protect public welfare including ecosystems, crops, and materials)

4For toxicology: non-carcinogens use RfD and hazard quotient (HQ = CDI / RfD); carcinogens use cancer slope factor; know that LD50 measures acute lethality, not chronic risk

5Green chemistry atom economy and the 12 principles appear in the Green Chemistry & Radiation section — memorize Principle 10 (Design for Degradation) and Principle 2 (Atom Economy)

Frequently Asked Questions

How many questions are on the NREP CES exam?

The CES exam contains 100 multiple-choice questions delivered online. All questions are scored and must be completed within a 2-hour time limit. The exam is intended to be closed-book and is administered through a link provided by email after application approval.

What score do I need to pass the NREP CES exam?

You must score at least 75% (75 of 100 questions correct) to pass the NREP CES exam. This is a higher passing threshold than the NREP REM (70%), reflecting the foundational science knowledge the CES credential requires.

What are the eligibility requirements for the NREP CES?

NREP requires either a bachelor's degree in an environmental or related field OR 3 years of environmental science work experience in lieu of the degree. Additionally, all candidates must have at least 3 years of work experience directly related to environmental science. Both requirements must be met; the experience used to substitute for education counts separately from the required work experience.

How much does the NREP CES exam cost?

The NREP CES requires a non-refundable application fee of $100 plus a separate examination fee of $200, for a total of approximately $300. An optional NREP e-book study guide is available for $150, and sample exam questions are available for $60.

What topics are covered on the NREP CES exam?

The CES exam covers seven content areas based on NREP's official content outline: Air (32%), Water (24%), Waste (24%), Human Health and Environmental Ethics (7%), Green Chemistry and Radiation (6%), Pesticides (6%), and Brownfields (1%). Air and Water together make up 56% of the exam.

How is the CES exam delivered?

The CES exam is an online examination delivered through a secure link sent by NREP after application approval. It is intended to be closed-book and completed within the 2-hour time limit. Proctored exam options (by mail to a third-party proctor) are available on request.

How does the CES differ from the NREP REM or REP?

The CES is NREP's entry-level environmental science credential — it tests broad foundational knowledge across all environmental science disciplines. The REM (Registered Environmental Manager) focuses on regulatory compliance management and requires 5 years of experience. The REP (Registered Environmental Professional) is the most senior NREP credential, requiring significant advanced education and experience. Many professionals earn CES early in their career and add REM or REP as they gain experience.

How often do I need to renew the NREP CES certification?

CES certification requires annual renewal with documented continuing education units (CEUs). Re-examination is not used for renewal — CEUs are the standard pathway. A renewal fee of $95 is required each year. Lapsed certifications may require reapplication or payment of a restoration fee.