N95 Respirators for Cardiovascular Protection During Wildfire Smoke Events

Wildfire smoke (WFS) impacted the Midwest/Eastern United States during 2023. Tens of millions of people were exposed to very unhealthy air. ¹ This represents a serious public health threat as such events are expected to increase and because WFS causes many health issues including cardiovascular morbidity and mortality. ² Fine particulate matter <2.5 μm (PM_2.5) is considered the principal culprit. Actions that individuals can take to protect their health, including wearing N95 respirators, have thus garnered considerable attention. ³ This is despite the fact that respirator use in the general public has undergone little scientific scrutiny. ² , ⁴ N95 respirators can reduce PM_2.5 exposure when properly fit and worn; however, their effectiveness in real‐world settings remains unknown. ⁴ Even assuming best‐case scenarios, the feasibility and costs required for large‐scale N95 respirator campaigns to protect the public during WFS events require careful consideration.

Data supporting this study are available from the corresponding author upon reasonable request. Institutional Review Board approval was not required. We estimated the number needed to treat (NNT₁) to prevent 1 atherosclerotic cardiovascular disease (ASCVD) event (fatal or nonfatal myocardial infarction or stroke) by using an N95 respirator based on data for the US adult population as well as intermediate‐ and high‐risk populations. We estimated a 20% increase in risk per day for ASCVD events during WFS exposure derived from the literature, which conforms with the risk of PM_2.5 at 200 μg/m³ (concentrations during recent events). ¹ , ² We estimated the NNT₁ under different assumptions of realistic N95 effectiveness, 50% (expected) or 75% (high), for reducing PM_2.5 exposure in the real‐world setting. The results of implausible 95% (perfect) effectiveness are also provided. We assumed a 1‐to‐1 ratio of exposure change‐to‐ASCVD event reduction. The NNT₁ was calculated as the inverse of the absolute ASCVD risk reduction over 1 day and 2 weeks. Finally, we estimated the costs of purchasing N95 respirators. We considered a scenario viable if the cost was similar to or less than average hospitalization stays for a myocardial infarction (≈$21 500). ⁵

The estimated NNT₁ and costs ranged widely across ASCVD risk profiles, respirator effectiveness, and time periods (Table). For the general public, over 1.2 million people would need to wear a respirator of 50% effectiveness to prevent 1 ASCVD in 1 day. Even over 2 weeks, the NNT₁ remains implausibly high. The realistic scenarios under which respirator use could viably prevent cardiovascular events from WFS exposure are for high‐risk adults and potentially for those at intermediate risk if high respirator effectiveness can be achieved.

Table  .

Estimated Number Needed to Treat and Costs of N95 Respirators to Prevent Atherosclerotic Cardiovascular Events During Wildfire Smoke Exposure

	General adult population	Adults aged 40–75 y (intermediate risk)	High risk or established ASCVD
Estimated number of Americans (millions)	260	65	25
Average 10‐y ASCVD risk	3%	10%	20%
Baseline daily ASCVD event rate (per million)*	8.22	27.40	54.79
ASCVD daily risk increase above baseline due to WFS	20%	20%	20%
Excess daily ASCVD event due to WFS (per million impacted)	1.64	5.48	10.96
Excess daily ASCVD events due to WFS if entire United States impacted	427	356	274
Expected effectiveness of N95 to reduce risk†	50%	50%	50%
Number of daily ASCVD events prevented (per million impacted by WFS)	0.82	2.74	5.48
Number of ASCVD events prevented if entire United States impacted by WFS (per d)	214	178	137
NNT1 (over 1 d)	1 216 667	365 000	182 500
NNT1 (over 2 wk)	86 905	26 071	13 036§
Cost‡, USD	$173 810	$52 143	$26 071§
High effectiveness of N95 to reduce risk	75%	75%	75%
Number of daily ASCVD events prevented (per million impacted by WFS)	1.23	4.11	8.22
Number of ASCVD events prevented if entire United States impacted by WFS (per d)	321	267	205
NNT1 ( over 1 d)	811 111	243 333	121 667
NNT1 (over 2 wk)	57 937	17 381§	8690§
Cost‡, USD	$115 873	$34 762§	$17 381§
Perfect effectiveness of N95 to reduce risk	95%	95%	95%
Number of daily ASCVD events prevented (per million impacted by WFS)	1.56	5.21	10.41
Number of ASCVD events prevented if entire United States impacted by WFS (per d)	406	338	260
NNT1 (over 1 d)	640 351	192 105	96 053
NNT1 (over 2 wk)	45 739	13 722	6861
Cost‡, USD	$91 479	$27 444	$13 722

ASCVD indicates atherosclerotic cardiovascular disease; NNT₁, number needed to treat to prevent 1 ASCVD event; USD, United States dollars; and WFS, wildfire smoke.

^*Daily event rate assumes the 10‐year risk is equally distributed throughout each day in the population.

^† The expected effectiveness is based upon real‐world scenarios for N95 use leading to 50% time‐average exposure reduction due to imperfect facial fit or seal, incomplete compliance, possible respirator damage, and not wearing for full 24 h (eg, indoors/sleep).

^‡ Assuming $2/mask, 1 mask per person per 2 wks.

^§ Most realistic scenarios for public health benefit with costs similar to average hospital stay for a myocardial infarction. ⁵

Wide‐scale use of N95 respirators by the general adult public during WFS events is not a viable approach to prevent cardiovascular events; whereas targeting high‐ (and possibly intermediate‐) risk adults may be feasible. Nonetheless, rapidly providing respirators to >13 000 high‐risk patients to prevent a single ASCVD event is a major undertaking requiring much foreplanning. It would also cost a similar amount as an average hospitalization stay for a myocardial infarction. ⁵ The conundrum is that the heightened risk from WFS generally lasts only a few days to weeks; hence, the absolute number of excess cardiovascular events is small. ² Without establishing a better plan of action, the promulgation for respirator use to the general public by the media, or public health authorities, poses more plausible harms (eg, unwarranted concerns/hysteria and expenditures) than benefits. Put into perspective, a 24‐hour exposure to WFS with a PM_2.5 concentration of 200 μg/m³ translates into an inhaled particle dose of ≈2 mg (similar to smoking 1–2 cigarettes). ⁴ While it is best to avoid exposures altogether, this dose is not an acute emergency for most adults.

Limitations of our analyses include less certainty regarding the cardiovascular toxicity of WFS particles and assumptions of deriving health benefits in direct relation to the degree of exposure reduction from respirator use (without potential harms). ⁴ There are no established criteria for the viability and cost‐effectiveness of respirator policies. We likely underestimated policy costs by not being able to account for all expenditures (eg, manpower); conversely, the full medical costs over months‐to‐year postmyocardial infarction are much larger than we estimated as the comparator. Certain individuals (eg, the elderly, and patients with ASCVD) are more susceptible to exposures than our uniform risk estimation. ² , ⁴ We also could not consider heart failure, arrhythmias, pulmonary diseases, or subclinical biomarkers. ⁴ However, >60% of deaths due to PM_2.5 are from ASCVD. ² , ⁴ Asthma is more germane to children in whom the safety of respirator use is not established. Adults with chronic obstructive pulmonary disease may not tolerate respirators and could experience more adverse effects. ⁴ Our results also do not represent a recommendation against wearing respirators per individual choice, nor that this action is entirely ineffective at protecting health. The findings ultimately highlight the need for personal‐level and civic/governmental action orregulations to prevent and mitigate future wildfire events. ² In conclusion, distribution of N95 respirators to higher‐risk individuals (not the general public) could in theory be a feasible policy.

Sources of Funding

This work was supported by grant funding from the National Institutes of Environmental Health (1R35ES031702‐01A1).

Disclosures

None.