Environment: Public Health & PHELO for MCCQE1

Introduction

Environmental health is a critical component of the MCCQE1 and falls under the domain of Population Health, Ethical, Legal, and Organizational Aspects of Medicine (PHELO). As a future Canadian physician, you are expected to identify, assess, and manage health risks associated with environmental hazards. This aligns with the CanMEDS Health Advocate role, requiring you to address determinants of health such as air quality, water safety, and housing conditions.

This guide covers high-yield environmental topics specifically tailored for MCCQE1 preparation, focusing on Canadian epidemiology, the Air Quality Health Index (AQHI), heavy metal toxicity, and occupational exposures.

Air Quality and Health

Air pollution is a significant contributor to cardiorespiratory morbidity in Canada. The MCCQE1 expects candidates to understand how to interpret air quality data and advise patients accordingly.

The Air Quality Health Index (AQHI)

Unlike the AQI used in the US, Canada uses the AQHI. It is a scale from 1 to 10+ designed to help Canadians understand what the air quality means for their health.

AQHI Risk Categories and Health Messages

*At-risk populations include individuals with asthma, COPD, heart disease, diabetes, young children, and the elderly.

Wildfire Smoke

With the increasing frequency of wildfires in Western and Northern Canada, smoke exposure is a high-yield topic.

• Primary Pollutant: PM2.5 (Fine particulate matter).
• Clinical: Exacerbation of Asthma/COPD, increased risk of myocardial infarction.
• Management: Stay indoors, use HEPA filters, N95 respirators (if necessary/fit-tested), and follow AQHI.

Water Quality and Safety

Water safety in Canada involves understanding both municipal treatment failures and specific issues facing rural and Indigenous communities.

Drinking Water Contaminants

Environmental Toxicology

Toxicology questions on the MCCQE1 often present as vague symptoms requiring a high index of suspicion based on environmental history.

1. Carbon Monoxide (CO) Poisoning

The “Silent Killer.” Common in Canadian winters due to furnaces, idling cars, or using propane heaters/BBQs indoors during power outages.

• Pathophysiology: CO binds to Hemoglobin with 200x affinity of O₂, forming Carboxyhemoglobin (HbCO), shifting the dissociation curve to the left (impairing O₂ unloading).
• Symptoms: Headache (most common), nausea, dizziness, confusion. “Cherry red lips” are a rare, late finding (do not rely on this).
• Diagnosis:
  • Standard Pulse Oximetry is NORMAL (cannot distinguish oxyhemoglobin from carboxyhemoglobin).
  • Gold Standard: Co-oximetry (ABG or VBG) to measure HbCO levels.
• Treatment: 100% Oxygen (non-rebreather mask). Hyperbaric oxygen for severe cases (pregnancy, coma, myocardial ischemia, HbCO >25%).

2. Lead Toxicity (Plumbism)

• Sources: Old paint (pre-1976), old pipes, imported pottery/spices, occupational (battery manufacturing).
• Pediatric Presentation: Developmental delay, behavioral changes, regression, anemia, abdominal pain.
• Adult Presentation: Peripheral neuropathy (wrist drop), abdominal pain (“lead colic”), hypertension.
• Investigation: Venous blood lead level (BLL).
  • CBC: Microcytic anemia, basophilic stippling on smear.
• Management: Removal of source. Chelation therapy (Succimer, EDTA, Dimercaprol) for severe toxicity.

3. Mercury

• Forms: Elemental (thermometers), Inorganic, Organic (Methylmercury - most toxic).
• Source: Consumption of predatory fish (shark, swordfish, fresh tuna).
• Canadian Guidelines: Health Canada advises pregnant women and children to limit consumption of high-mercury fish.
• Symptoms: Paresthesias, ataxia, visual field constriction (Minamata disease).

Climate Change and Vector-Borne Diseases

Climate change is expanding the habitat of vectors in Canada, moving northward.

Lyme Disease

• Vector: Ixodes scapularis (Blacklegged tick).
• Geography: Southern Ontario, Quebec, Manitoba, Nova Scotia, Southern BC.
• Agent: Borrelia burgdorferi.
• Clinical Stages:
  1. Early Localized: Erythema Migrans (Bull’s eye rash).
  2. Early Disseminated: AV block, Bell’s palsy, migratory arthralgias.
  3. Late: Chronic arthritis, encephalopathy.
• Prophylaxis: Single dose Doxycycline (if tick attached >36h and endemic area).

West Nile Virus

• Vector: Culex mosquito.
• Reservoir: Birds.
• Clinical: Mostly asymptomatic. West Nile Fever (flu-like). Neuroinvasive disease (Meningitis/Encephalitis/Flaccid Paralysis) in elderly/immunocompromised.

Occupational Environmental Health

Taking an exposure history is a mandatory skill for the MCCQE1. Use the CH2OPD2 mnemonic.

Occupational Lung Diseases

Canadian Guidelines

For MCCQE1, adhere to recommendations from Health Canada and the Canadian Task Force on Preventive Health Care (CTFPHC).

1. Lead Screening:

   • Universal screening is NOT recommended in Canada.
   • Targeted screening for children with risk factors (e.g., recent immigrant/refugee, pica, living in homes built before 1960 with peeling paint).
   • Reference: Rourke Baby Record.

2. Radon:

   • Health Canada recommends all homes be tested for radon (long-term test: 3 months).
   • Radon is the 2nd leading cause of lung cancer in Canada (after smoking).
   • Action level: >200 Bq/m³.

3. Sun Safety:

   • UV Index 3 or higher: protection required.
   • No sunscreen for infants <6 months (keep out of direct sun).

Key Points to Remember for MCCQE1

• CO Poisoning: Pulse ox is normal. Order Co-oximetry. Treat with 100% O₂.
• Asbestos: Pleural plaques are the hallmark. Bronchogenic carcinoma is the most common cancer; Mesothelioma is the most specific.
• Indigenous Health: Be aware of the high prevalence of boil water advisories and housing mold issues in First Nations communities as a structural determinant of health.
• AQHI: Use the 1-10+ scale. Know that “High Risk” (7-10) requires reducing strenuous outdoor activities.
• History Taking: Always ask: “Do your symptoms improve when you are away from work or home?”

Sample Question

# Sample Question

A 55-year-old male presents to the emergency department in January complaining of a persistent headache, nausea, and general malaise for the past 12 hours. He states that his wife has been feeling similarly “flu-like.” They live in an older home and recently started using a gas space heater in the basement because their furnace has been acting up. He has a history of hypertension. Vital signs are: BP 145/90 mmHg, HR 105 bpm, RR 22/min, Temp 37.1°C, and O₂ saturation 98% on room air. Physical examination reveals no focal neurological deficits and normal lung auscultation.

Which one of the following is the most appropriate next step in management?

• A. Administer acetaminophen and discharge with viral precautions
• B. Order a CT scan of the head
• C. Obtain an arterial blood gas with co-oximetry
• D. Perform a lumbar puncture
• E. Initiate 100% oxygen via nasal prongs at 2 L/min

Explanation

The correct answer is:

• C. Obtain an arterial blood gas with co-oximetry

Explanation: This clinical scenario is highly suggestive of Carbon Monoxide (CO) poisoning. The key clues are:

1. Epidemiology/Season: January (winter), use of a gas space heater, furnace issues.
2. Symptoms: Headache, nausea, malaise (often misdiagnosed as a viral illness).
3. Multiple victims: The wife is also symptomatic, which strongly points to an environmental cause rather than an infectious one (though viral spread is possible, the environmental clues are stronger).
4. Vital Signs: Tachycardia is common. Crucially, the O₂ saturation is normal (98%). Standard pulse oximetry cannot distinguish between oxyhemoglobin and carboxyhemoglobin.

To confirm the diagnosis, you must measure the Carboxyhemoglobin (COHb) level. This requires co-oximetry, which can be done via ABG or VBG.

• Option A: Discharging a patient with potential CO poisoning back to the toxic environment could be fatal.
• Option B: CT head might be considered for a headache, but the environmental clues make CO poisoning more likely. A CT is not the initial priority.
• Option D: Lumbar puncture is for meningitis. While he has a headache, he is afebrile and has no nuchal rigidity mentioned.
• Option E: Treatment for CO poisoning is 100% oxygen, but it must be delivered via a non-rebreather mask (high flow), not nasal prongs at 2 L/min, to effectively reduce the half-life of COHb. Furthermore, diagnosis (C) typically occurs concurrently with initiating high-flow O2 in the ER setting, but C is the diagnostic step required.

References

1. Health Canada. (2023). Air Quality Health Index. Retrieved from canada.ca 
2. Medical Council of Canada. (2023). MCCQE Part I Objectives: Population Health.
3. Public Health Agency of Canada. (2023). Lyme disease: For health professionals.
4. Abelsohn, A., & Sanborn, M. (2010). Lead and children: Clinical management for family physicians. Canadian Family Physician, 56(6), 531–535.
5. Rourke Baby Record. (2020). Evidence-based infant/child health maintenance guide.